Supplementary Information 3: Mass Spectrometry Studies of Candidate 7500

Candidate 7500’s multiple peaks in the range m/z 7-8000 in MALDI-TOF MS are variant ions of a unitary polypeptide, namely sSgII-70. This view informs the following analysis, which will argue that the observed heterogeneity in MALDI MS is due to (a) C-terminal truncation (i.e. loss of aa residues from the C-terminus by peptide bond cleavage) and (b) water losses in variable degrees.
The core data fittings involve specimen MALDI spectra from ovine and bovine purification runs, analysing a dozen or so MS peaks from each species in the range m/z 7-8000. C-terminal and N-terminal truncations have been tried, based on the SgII-70 sequence deductions, with C-terminal truncation favoured for tightness of fit between observations and matches. The data fitting exercise is then followed by a series of bespoke analyses.