Spherulation-specific family 4. This protein is found in bacteria, archaea and eukaryotes. Proteins in this family are typically between 250 and 398 amino acids in length. There is a conserved NPG sequence motif and there are two completely conserved G residues that may be functionally important. Starvation will often induce spherulation - the production of spores - and this process may involve DNA-methylation. Changes in the methylation of spherulin4 are associated with the formation of spherules, but these changes are probably transient. Methylation of the gene accompanies its transcriptional activation, and spherulin4 mRNA is only detectable in late spherulating cultures and mature spherules. It is a spherulation-specific protein.
sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA), similar to mammalian ATP2A1-3/SERCA1-3. SERCA is a transmembrane (Ca2+)-ATPase and a major regulator of Ca(2+) homeostasis and contractility in cardiac and skeletal muscle. It re-sequesters cytoplasmic Ca(2+) to the sarco/endoplasmic reticulum store, thereby also terminating Ca(2+)-induced signaling such as in muscle contraction. Three genes (ATP2A1-3/SERCA1-3) encode SERCA pumps in mammals, further isoforms exist due to alternative splicing of transcripts. The activity of SERCA is regulated by two small membrane proteins called phospholamban and sarcolipin. This subfamily belongs to the P-type ATPases, a large family of integral membrane transporters that are of critical importance in all kingdoms of life. They generate and maintain (electro-) chemical gradients across cellular membranes, by translocating cations, heavy metals and lipids, and are distinguished from other main classes of transport ATPases (F- , V- , and ABC- type) by the formation of a phosphorylated (P-) intermediate state in the catalytic cycle.
