Variation in Orthologous Shell-Forming Proteins Contribute to Molluscan Shell Diversity

Abstract

Despite the evolutionary success and ancient heritage of the molluscan shell, little is known about the molecular details of its formation, evolutionary origins, or the interactions between the material properties of the shell and its organic constituents. In contrast to this dearth of information, a growing collection of molluscan shell-forming proteomes and transcriptomes suggest they are comprised of both deeply conserved, and lineage specific elements. Analyses of these sequence data sets have suggested that mechanisms such as exon shuffling, gene co-option, and gene family expansion facilitated the rapid evolution of shell-forming proteomes and supported the diversification of this phylum specific structure. In order to further investigate and test these ideas we have examined the molecular features and spatial expression patterns of two shell-forming genes (Lustrin and ML1A2) and coupled these observations with materials properties measurements of shells from a group of closely related gastropods (abalone). We find that the prominent “GS” domain of Lustrin, a domain believed to confer elastomeric properties to the shell, varies significantly in length between the species we investigated. Furthermore, the spatial expression patterns of Lustrin and ML1A2 also vary significantly between species, suggesting that both protein architecture, and the regulation of spatial gene expression patterns, are important drivers of molluscan shell evolution. Variation in these molecular features might relate to certain materials properties of the shells of these species. These insights reveal an important and underappreciated source of variation within shell-forming proteomes that must contribute to the diversity of molluscan shell phenotypes.

The Biomineralization Proteome: Protein Complexity for a Complex Bioceramic Assembly Process

“…The transcriptome or RNA expression levels in these organisms are also being explored with the goal of understanding the timing of transcription, translation, and protein secretion as it pertains to the events occurring in the biomineralizing matrix . What has emerged from these studies…”

“…The mollusk, of the phyla Lophotrochozoa, is one of the most diverse groups on Earth (>50 000 species) that creates a protective outer shell that has a high degree of organization, much more so than bone, and this system has been intensely studied for several decades . In most bivalve mollusks, the shell consists of three layers (Figure ): 1) the periostracum, which is a thin organic coating or “skin” comprising the outermost layer of the shell; 2) the prismatic layer, the outermost mineral layer of the shell that comprises organized parallel rods of single crystal calcite, a calcium carbonate polymorph; 3) nacre layer, or the innermost mineral layer of the shell that comprises hexagonal tablets of single crystal aragonite, a calcium carbonate polymorph, that are arranged in a “brick‐and‐mortar” organization . Both the prismatic and nacre mineral particles are coated with a protein‐containing film or layer that is termed the framework layer and this will be discussed in more detail below .…”

“…The outer epithelial layer of the mantle secretes biomineralization proteins and the calcium carbonate mineral phase into a fluid‐filled space termed the extrapallial space. This space separates the mantle epithelium from the nacre layer . Note that Figure applies to mollusks which possess a bilayer prismatic–nacre shell; however, in some mollusks, there is only a single mineral layer, either a prismatic or a nacre one…”

“…There is definitive evidence for a temporal and spatial secretion of mollusk shell proteins during the development of the shell, which indicates that certain proteins must be present in definitive regions and at specific developmental times . The shell secretome involves many proteins, some of which are post‐translationally modified (i.e., phosphorylated, glycosylated), and many of these proteins contain unfolded or intrinsically disordered sequence regions . Once again, the presence of intrinsic disorder indicates that these proteins are conformationally unstable and thus prone to interactions with either other proteins and/or the mineral phase .…”

“…Within these genomes lurk mineralization‐associated genes. Since many biomineral‐associated proteins are extracellular and thus subjected to post‐translational modifications, these genomic studies have been augmented by proteomic studies that identify gene products that are phosphorylated, sulfated, glycosylated, enzymatically cleaved, or otherwise chemically modified at side‐chain groups . The challenge that we are now facing is to identify these mineralization‐associated proteins, their structure, synthesis, secretion, and temporal/regional localization, and most importantly, their purpose or function.…”