mots-c mastery course
Unit 2 of 10

chemistry and genetic origin

16 amino acids encoded by your mitochondria

a peptide written into a ribosomal RNA gene

MOTS-c is 16 amino acids long. Its sequence (MRWQEMGYIFYPRKLR) is encoded by a short open reading frame hidden inside MT-RNR1, the mitochondrial gene that classical biology taught as a structural rRNA gene with no protein-coding role at all. This unit covers the chemistry, the gene location, and why this arrangement is biologically remarkable.

The mitochondrial genetic code differs slightly from the nuclear one. That detail matters here -- the same DNA letters read in the nucleus would code a different protein than what your mitochondria actually translate.

chemistry at a glance

key molecular and genetic parameters.

16 residues
MRWQEMGYIFYPRKLR -- full peptide sequence
~2,174 Da
monoisotopic molecular weight
MT-RNR1
mitochondrial 12S rRNA gene
~51 nt
length of the encoding ORF (start + stop)

sequence and ORF explorer

click any residue to see its chemistry and its role in the peptide. the takeaway is that MOTS-c is a 16-residue peptide where a small number of charged and aromatic residues do most of the functional work -- which is why a single-residue swap like K14Q can shift bioactivity in measurable ways.

MOTS-c residue viewer

key terms

definitions you will encounter throughout this unit.

Oopen reading framemechanism
A stretch of DNA or RNA that begins with a start codon and ends with a stop codon, with no internal stops, so it can in principle be read as a protein. Abbreviated ORF. MOTS-c is encoded by a very short ORF (about 51 nucleotides) tucked inside a much larger rRNA gene.
RrRNAmolecule
Ribosomal RNA. The catalytic and structural RNA that, together with a set of proteins, forms a ribosome -- the cellular machine that builds proteins. Mitochondria carry their own ribosomes and their own rRNAs, called 12S and 16S. MOTS-c is encoded inside the gene for 12S rRNA (MT-RNR1).
Ccodonmechanism
A three-nucleotide unit of the genetic code that specifies one amino acid (or a start/stop signal). The standard genetic code uses 64 codons. The mitochondrial genetic code reassigns a few of them, which matters when reading mitochondrial ORFs.
Mmicroproteinpeptide
A small protein translated from a short open reading frame, often under 100 amino acids. Older gene-finding software ignored ORFs this short by default, which is why MOTS-c and other microproteins went unnoticed for decades.
Hheteroplasmymechanism
The situation where a cell contains a mix of mitochondrial DNA molecules -- some carrying a variant, some not. mtDNA copy number is high (hundreds to thousands per cell), so heteroplasmy is common. It matters here because the K14Q variant in MOTS-c (covered in unit 8) is often present at variable heteroplasmy in carriers.
SsORFmechanism
Small open reading frame. The same concept as a regular ORF but explicitly short -- the term is used to flag the class of microprotein-coding sequences that classical gene annotation missed. MOTS-c is a textbook example of an active sORF product.

the sequence -- simple version first

a 16-residue chain, written into a gene that has a different day job.

the peptide itself

MOTS-c is 16 amino acids long. Using single-letter abbreviations, the sequence reads MRWQEMGYIFYPRKLR. Every one of these letters is a standard amino acid; nothing about the peptide is chemically exotic. What is unusual is where the recipe comes from.

where the recipe lives

The mitochondrial DNA in every one of your cells carries a short stretch of code -- about 51 nucleotides -- inside the MT-RNR1 gene. That gene's official job is to produce the structural RNA of the small subunit of the mitochondrial ribosome.

two readings of one sequence

The MOTS-c open reading frame is layered on top of that, in a different reading frame, like a poem hidden inside a manual. When the cell needs MOTS-c, the same DNA gets read with a different alignment and a peptide comes out instead of an rRNA.

Aadvanced: the mitochondrial genetic code, in detailterm
The standard genetic code, used by nuclear genes, assigns one specific amino acid to each of the 64 possible three-letter codons. Mitochondria use a slightly different table. In vertebrate mitochondria, UGA -- which is normally a stop codon -- is reassigned to tryptophan. AUA is reassigned from isoleucine to methionine. AGA and AGG are reassigned from arginine to stop codons (in mammals). These reassignments only apply to genes translated by the mitochondrial ribosome. When reading the MOTS-c ORF off the mtDNA sequence, you need the mitochondrial code -- using the standard code would read out a different (incorrect) peptide and would miss the real start codon. This is one reason microproteins inside mitochondrial rRNAs went undetected for so long: standard gene-finding tools were trained on the nuclear code.
Aadvanced: reading-frame strategyterm
Any DNA sequence can be read in three possible reading frames on each strand (six in total). Most of those reading frames will hit stop codons quickly and produce nothing useful. The MOTS-c open reading frame uses a different reading frame than the one the ribosome uses for the 12S rRNA functional role. So the same nucleotide sequence carries two different functional outputs at once: in one reading frame it folds into part of the ribosome; in another it codes a 16-residue signaling peptide. This is called overlapping coding and is rare in nuclear genes but well documented in compact genomes (viruses, bacteria, and organelle DNAs).

how MOTS-c compares to other peptides you know

size and origin context.

MOTS-c

  • 16 residues
  • encoded by mitochondrial DNA (MT-RNR1, 12S rRNA)
  • uses the mitochondrial genetic code
  • found in circulation; acts on muscle, liver, adipose, nucleus
  • activates AMPK via AICAR accumulation

Humanin

  • 24 residues
  • encoded by mitochondrial DNA (MT-RNR2, 16S rRNA)
  • also uses the mitochondrial genetic code
  • found in circulation; acts on neurons and immune cells
  • neuroprotective and cytoprotective, not metabolic

Insulin

  • 51 residues (two chains, A and B)
  • encoded by nuclear DNA (INS gene, chromosome 11)
  • uses the standard nuclear genetic code
  • secreted by pancreatic beta cells
  • binds the insulin receptor; classical endocrine hormone
Do not confuse MOTS-c length with how powerful it is. 16 residues is short by hormone standards but MOTS-c modulates a master energy-sensor cascade. Peptide length and biological reach are uncorrelated.