Major matrix protein of the sea urchin embryo spicule. The function of the matrix proteins is to direct crystal growth in certain orientations and inhibit growth in others. Expressed specifically in the micromere/primary mesenchyme cells (PMC) lineage. Detected at hatching blastula, it remains at a certain level from the mesenchyme blastula to the pluteus stages. The repetitive domain may provide a calcite binding matrix. Belongs to the SM50 family.