Primary structure and lens-specific expression of genes for an intermediate filament protein and a β-tubulin in cephalopods

Abstract

Intermediate filament (IF) protein and tubulin cDNAs of cephalopod eye lenses were cloned and sequenced. The rod regions of the deduced IF proteins of the squid and octopus were more similar (68% identical) than were head (33% identical) and tail (40% identical) regions. The rod sequences were closer to squid neuronal IF protein (39% identical) than to any other known IF protein. There was only 31% identity between the rod regions, 21–30% identity between the head regions and 23–32% identity between the tail regions of the present IF proteins of cephalopods and other invertebrates. The rod regions of the cephalopod IF proteins contained the 6 heptads characteristic of nuclear lamins, consistent with an evolutionary relationship between IF proteins and lamins. The present octopus α-tubulin was 93% and β-tubulin was 87% identical to the corresponding tubulins of insects and vertebrates. SDS-PAGE and peptide sequencing indicated that the order of abundance of the cephalopod lens cytoskeletal proteins was IF proteins, actin and tubulins. Northern blot hybridization revealed a 4 kb mRNA for the octopus IF protein and 2.9 and 7.3 kb mRNAs for the squid IF protein; the α-tubulin mRNA was about 1.8 kb in the octopus and squid, and the β-tubulin mRNA was about 2.8 kb in the octopus. The α-tubulin mRNA was present in all tissues examined; by contrast, the present β-tubulin and IF protein mRNAs appeared specialized for lens expression.