Physicochemical characterization of lens proteins of the squid Nototodarus gouldi and comparison with vertebrate crystallins.

The main water-soluble proteins of squid lens (S-crystallins) have a molecular weight of 60 000, a sedimentation coefficient s020,w of 5.2 S, 20-30% alpha-helical secondary structure, and an unusually high methionine content (12%). The subunits of Mr 30 000 (major) and Mr 27 000 (minor) have related N-terminal amino acid sequences, but a very heterogeneous charge distribution with predominantly basic isoelectric points. Higher-Mr aggregates have similar secondary/tertiary structure and amino acid composition, but contain additional acidic subunits and Mr 35 000-40 000 subunits. S-crystallins resemble vertebrate beta-crystallins in their quaternary structure, and their N-terminal sequence shows analogy with the first 19 residues of calf beta/gamma-crystallin folding units. In the urea-soluble and urea-insoluble lens fractions polypeptides of Mr 58 000 and 80 000, respectively, predominate, which presumably correspond to the main cytoskeleton and membrane proteins. Water-soluble lens components of less than Mr 2000 were isolated which have ultraviolet absorption maxima at 327 and 370 nm.