A PROTEIN SIMILAR TO THE 67-KDA LAMININ-BINDING PROTEIN AND P40 IS PROBABLY A COMPONENT OF THE TRANSLATIONAL MACHINERY IN URECHIS-CAUPO OOCYTES AND EMBRYOS

Oocytes of the echiuroid, Urechis caupo, contain an abundant maternal mRNA that encodes a protein very similar to LBP/p40, originally identi fied as a non-integrin 67 kDa laminin binding protein. We have sequenc ed the Urechis caupo mRNA for LBP/p40, and a similar mRNA from the Haw aiian sea urchin, Tripneustes gratilla. Both of the encoded proteins, as well as LBP/p40 proteins from other sources, share significant homo logy in the amino 2/3 of the proteins, but diverge extensively at the carboxyl ends, LBP/p40 protein is present in growing and in full-grown U, caupo oocytes. The protein concentration remains constant for the first 48 hours of embryogenesis and then begins to decline. In sucrose gradients run with homogenates from coelomocytes, oocytes, and early embryos, all of the LBP/p40 protein appears to be associated with eith er polysomes or free 40 S ribosomal subunits. In later embryos, an inc reasing proportion of the protein is found in the soluble fraction. Im munohistochemistry indicates that LBP/p40 is uniformly distributed in early U, caupo embryos, with no localization at the cell surface. In l ater embryos LBP/p40 is localized in specific parts of the embryo whic h may correspond to neural tissue.