Structure, Expression, and Extracellular Targeting of PM27, a Skeletal Protein Associated Specifically with Growth of the Sea Urchin Larval Spicule

Abstract

The PM27 gene encodes a sea urchin skeletal protein. Both the transcript and encoded protein appear at the mesenchyme blastula stage and are restricted to the primary mesenchyme cell (PMC) lineage throughout development. Transgenic expression of PM27 promoter constructs demonstrates that this cell specificity is regulated at the level of transcription. The PM27 sequence predicts a nonglycosylated secretory product of 27 kDa in mature form. The N-terminal "repeat" domain of the deduced amino acid sequence consists of a series of tandem repeats and shares both sequence and predicted structural similarities with several fiber-forming proteins. The C-terminal "lectin" domain is similar to the C-class lectins. Antisera against either domain of PM27 detect two major proteins in embryo extracts, with apparent molecular weights of 27 and 30 kDa. Immunolocalization in whole embryos demonstrates that PM27 antigen is produced uniformly and exclusively by PMCs through the early prism stage and that this specificity is further restricted during skeletogenesis to a subpopulation of PMCs associated with the growing tips of the spicules. It is secreted to the skeletal compartment and accumulates predominantly at the advancing mineralizing surface of the spicule tips. As the spicules elongate PM27 protein disappears from the more mature mid-shaft regions. The observed characteristics of PM27 are consistent with a role in the regulation or execution of skeletal growth, as opposed to maintenance or structural integrity of the spicules.