Many of the structural proteins of ocular lenses, commonly referred to as
crystallins, are identical to specific
enzymes or the result of a recent
gene duplication (Piatigorsky, J., and Wistow, G. (1991) Science 252, 1078-1079). One such
enzyme,
aldehyde dehydrogenase (
ALDH), has been recruited as a lens
crystallin in certain mammals (Wistow, G., and Kim, H. (1991) J. Mol. Evol. 32, 262-269) and cephalopods (Tomarev, S., Zinovieva, R., and Piatigorsky, J. (1991) J. Biol. Chem. 266, 24226-24231). We report here that a transparent tissue,
derived from
muscle but functioning as a lens in the light-emitting
organ of a
squid, Euprymna scolopes, shows striking biochemical convergence with the epidermally
derived ocular lenses of some mammals and cephalopods. In the light
organ lens of E. scolopes, an ALDH-like protein is the predominant molecular component. The typical muscle-specific proteins are replaced as the dominant species by a protein composed of 54-kDa
subunits. This protein, which we designate as L-crystallin, constitutes approximately 70% of the total
soluble protein of the light
organ lens. The
amino acid sequences of three
peptides of L-crystallin (approximately 9% of the total protein) showed 54.5%
sequence identity with human
cytosolic ALDH. Using
polyclonal antiserum made against L-crystallin, we found that it is present in low abundance in other tissues of the
squid, including
muscle and the
ocular lens. This
polyclonal antiserum also cross-reacted with the ALDH-like
crystallins found in the ocular lenses of certain mammals and cephalopods. L-Crystallin showed no
ALDH activity, which is similar to several other
enzyme/
crystallins, including
ALDH/eta-crystallin (Wistow, G., and Kim, H. (1991) J. Mol. Evol. 32, 262-269). The characteristics of this muscle-derived lens are evidence that a common biochemical basis underlies transparency and that certain proteins may possess properties that promote their
selection as lens structural proteins.
