Alternative splicing of an region generates multiple amino-terminal PEVK-like isoforms of Drosophila projectin

Drosophila projectin is an extremely large protein found within the muscle sarcomeric unit, parallel with the actin and myosin filaments. Projectin ha s been suggested as the elastic component of C-filaments in insect indirect flight muscles, which is consistent with its localization from the Z band to the tip of the A band in these muscles. Here, we describe the completion of the projectin sequence analysis, which defines projectin as a 1 MDa pro tein, composed of 39 immunoglobulin and 39 fibronectin III domains. This an alysis led also to the identification of a domain rich in the amino acids P , E, V and K within the NH2 terminus of projectin. The length of the projec tin PEVK-like region varies from 100 to 624 amino acid residues, following a complex pattern of alternative splicing events. PEVK domains were first i dentified in vertebrate titin and they have been associated with the elasti city of the protein. The PEVK-like domain of the projectin isoforms in indi rect flight muscles may contribute to the elastic function of the C-filamen ts. The synchronous projectin isoforms contain a PEVK-like region, and the possible non-elastic function(s) of this domain in synchronous muscles are discussed. (C) 2001 Academic Press.