Recombinant transgelin-like protein 1 from Mytilus shell induces formation of CaCO₃ polymorphic crystals in vitro

Transgelin is an actin cross-linking/gelling protein of the calponin family, which is associated with actin stress fibres, cell motility, adhesion and the maintenance of cell morphology. Transgelin-like proteins (TLPs) have also been identified as shell matrix proteins (SMPs) in several mollusc species; however, the functions of TLPs in biomineralization remain unknown. Transgelin-like protein 1 (TLP-1) was previously identified from the shell of Mytilus coruscus as a novel 19 kDa SMP with a calponin homology (CH) domain. To understand the role of TLP-1 in shell formation, the expression level and localization of the TLP-1 gene in biomineralization-related tissues were determined in this study. Furthermore, recombinant TLP-1 was expressed in a prokaryotic expression system with codon optimization, and an anti-rTLP-1 antibody was prepared based on the expressed recombinant TLP-1 (rTLP-1) protein. In vitro, rTLP-1 induced the formation of CaCO₃ polymorphic crystals with distinct morphologies and inhibited crystallization rate and crystal interactions. Immunohistochemical, immunofluorescence, and pull-down analyses using the anti-rTLP-1 antibody revealed the specific locations of TLP-1 in biomineralization-related tissues and shell myostracum layer, and suggested the existence of a possible TLP-1 interaction network in the shell matrix. Our results are beneficial for understanding the functions of TLP-1, particularly through its CH domain, during shell mineralization.