MOTS-c vs SS-31: two mitochondrial peptides compared

Where do MOTS-c and SS-31 come from?

MOTS-c is a natural peptide encoded in mitochondrial DNA; SS-31 is a designed, synthetic peptide. The two peptides are often grouped together because both are studied in mitochondrial research, but they have nothing in common in origin. MOTS-c is natural: Lee and colleagues described it in 2015 as the product of a short open reading frame inside the 12S rRNA gene of mitochondrial DNA. Its sixteen residues, MRWQEMGYIFYPRKLR, are encoded by the mitochondrial genome itself, which is why it is grouped with humanin as a mitochondrial-derived peptide.

SS-31 is designed. It is one of the Szeto-Schiller peptides developed at Cornell around a motif of alternating aromatic and basic residues, and it is four residues long: D-Arg-Dmt-Lys-Phe-NH2, where Dmt is 2',6'-dimethyltyrosine. The D-arginine, the non-natural Dmt residue and the amidated C-terminus are all synthetic choices. It is also known as elamipretide, the name used in clinical trials.

The size difference matters in the lab. MOTS-c has a molecular weight of about 2174.6 g/mol and two methionines; SS-31 is about 639.8 g/mol. The identity tables on the MOTS-c and SS-31 pages give the formula, CAS number and PubChem record for each.

Where are MOTS-c and SS-31 proposed to act?

MOTS-c is studied as a messenger from the mitochondrion to the nucleus, and SS-31 as a membrane-binding compound that works on the mitochondrion itself. The MOTS-c literature describes a signalling peptide that acts outside the organelle it comes from. In the founding paper, MOTS-c inhibited the folate cycle and the de novo purine biosynthesis linked to it in cultured cells, which was proposed to lead to AMPK activation, with skeletal muscle identified as its apparent primary target tissue in mice. Kim and colleagues later reported that under metabolic stress, such as glucose restriction, MOTS-c moved from the cytoplasm into the nucleus in an AMPK-dependent manner and regulated nuclear genes, including genes with antioxidant response elements.

The SS-31 literature describes a molecule that stays at one place inside the mitochondrion. Zhao and colleagues reported that the Szeto-Schiller peptides concentrate roughly 1000-fold in the inner mitochondrial membrane. Birk and colleagues then used a fluorescent analogue to report that SS-31 binds cardiolipin, a phospholipid of that membrane, and that the SS-31/cardiolipin complex inhibited the peroxidase activity of cytochrome c. The proposed account is structural: keep cardiolipin and cristae intact, and electron transport is protected.

The two accounts are not competing explanations of the same effect, so the literature gives no basis for ranking one against the other.

What models have MOTS-c and SS-31 been studied in?

Outside the clinic, both have been studied mainly in cultured cells and rodents, and each body of work clusters around its originating laboratories. MOTS-c work is mainly in cultured cells and mice. Cell studies cover the folate cycle, AMPK activation, nuclear translocation under glucose restriction and osteoclast differentiation. Mouse studies include high-fat-diet models of insulin resistance, plasma metabolomics in diet-induced obese mice and an ovariectomy model of bone loss. Much of the foundational work comes from a small number of laboratories at the University of Southern California.

SS-31 work spans neuronal cell lines, isolated mitochondria, an ex vivo heart reperfusion model, rat renal ischaemia-reperfusion, and mouse models of angiotensin II-induced cardiac hypertrophy and Gαq-overexpression heart failure. Much of the early mechanistic work comes from the Cornell group that designed the peptides. Both bodies of work cluster around their originating laboratories, which is worth keeping in mind when reading either.

Have MOTS-c or SS-31 been tested in clinical trials?

SS-31, as elamipretide, has reached human trials; MOTS-c work remains preclinical. This is the largest difference between the two. MOTS-c has not been taken into published clinical trials; the work described on this page is from cells and mice.

SS-31, as elamipretide, has. In the phase 3 MMPOWER-3 trial in primary mitochondrial myopathy, Karaa and colleagues reported that it did not meet its primary endpoints of 6-minute walk distance and fatigue score. In a smaller crossover trial in Barth syndrome, a disorder of cardiolipin metabolism, Reid Thompson and colleagues reported that neither primary endpoint was met in the double-blind part, with changes reported only in the open-label extension. As our product page notes, the US FDA granted elamipretide accelerated approval for Barth syndrome in September 2025 on the basis of knee extensor strength, with a confirmatory trial required.

Having clinical trials is not the same as having positive ones. What the trials do show is that elamipretide has been examined in people in controlled settings, with published endpoints, which MOTS-c has not. That is a statement about the size and kind of the literature, not about what either compound does.

What does research on MOTS-c and SS-31 not show?

Neither body of work establishes a general effect in healthy people. The MOTS-c pathways are proposals drawn from individual cell and mouse models, and none has been established in humans. The SS-31 trials were in two specific rare mitochondrial conditions, and their primary endpoints were not met; they do not extend to other conditions or to healthy subjects.

We found no published study comparing the two peptides directly, in any model. Claims elsewhere that one is stronger, or that they combine well, are not supported by the papers cited here.

What do the MOTS-c and SS-31 certificates show?

Only MOTS-c has a certificate so far, and it reports purity, not identity. MOTS-c has a certificate of analysis for lot ABL-MOT-2602, issued by Janoshik, which reports purity by HPLC. It does not include an identity test such as mass spectrometry. The molecular formula printed on it counts one sulfur atom where the sequence has two methionines; we show the corrected formula, C101H152N28O22S2, which matches the certificate's own stated molecular weight. The COA library lists the certificate with that correction, and our guide to reading a peptide COA covers why a printed formula is reference information rather than a measurement.

SS-31 does not have a certificate yet, and the COA library lists it as in progress. Until one is published, its identity figures come from PubChem and have not been checked against a certificate. For what identity testing adds over purity alone, see HPLC vs mass spec: what each test proves.

References

1. Lee C, Zeng J, et al. (2015). The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance. Cell Metab. [RODENT] PubMed record for reference 1, Lee 2015
2. Kim KH, Son JM, et al. (2018). The Mitochondrial-Encoded Peptide MOTS-c Translocates to the Nucleus to Regulate Nuclear Gene Expression in Response to Metabolic Stress. Cell Metab. [IN VITRO] PubMed record for reference 2, Kim 2018
3. Zhao K, Zhao GM, Wu D, et al. (2004). Cell-permeable peptide antioxidants targeted to inner mitochondrial membrane inhibit mitochondrial swelling, oxidative cell death, and reperfusion injury. J Biol Chem. [IN VITRO] PubMed record for reference 3, Zhao 2004
4. Birk AV, Liu S, Soong Y, et al. (2013). The mitochondrial-targeted compound SS-31 re-energizes ischemic mitochondria by interacting with cardiolipin. J Am Soc Nephrol. [RODENT] PubMed record for reference 4, Birk 2013
5. Karaa A, Bertini E, Carelli V, et al. (2023). Efficacy and Safety of Elamipretide in Individuals With Primary Mitochondrial Myopathy: The MMPOWER-3 Randomized Clinical Trial. Neurology. [HUMAN CLINICAL] PubMed record for reference 5, Karaa 2023
6. Reid Thompson W, Hornby B, Manuel R, et al. (2021). A phase 2/3 randomized clinical trial followed by an open-label extension to evaluate the effectiveness of elamipretide in Barth syndrome, a genetic disorder of mitochondrial cardiolipin metabolism. Genet Med. [HUMAN CLINICAL] PubMed record for reference 6, Reid 2021
7. Ming W, Lu G, et al. (2016). Mitochondria related peptide MOTS-c suppresses ovariectomy-induced bone loss via AMPK activation. Biochem Biophys Res Commun. [RODENT] PubMed record for reference 7, Ming 2016
8. Benayoun BA, Lee C (2019). MOTS-c: A Mitochondrial-Encoded Regulator of the Nucleus. Bioessays. [REVIEW] PubMed record for reference 8, Benayoun 2019
9. Dai DF, Chen T, Szeto H, et al. (2011). Mitochondrial targeted antioxidant Peptide ameliorates hypertensive cardiomyopathy. J Am Coll Cardiol. [RODENT] PubMed record for reference 9, Dai 2011
10. Szeto HH (2014). First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics. Br J Pharmacol. [REVIEW] PubMed record for reference 10, Szeto 2014

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