Maize cap1 encodes a novel SERA-type calcium-ATPase with a calmodulin- binding domain

A cDNA (CAP1) isolated from maize roots shares sequence identity with genes encoding P-type Ca²⁺-ATPases and restores the growth phenotype of yeast mutants defective in Ca²⁺-pumps. CAP1 was transcribed and translated in the yeast mutant. Furthermore, the membrane-integrated product formed a Ca²⁺-dependent phosphorylated intermediate and supported Ca²⁺ transport. Although CAP1 shares greater sequence identity with mammalian 'endoplasmic reticulum-type' Ca²⁺-pumps, it differs from these genes by having features of calmodulin (CaM)-regulated Ca²⁺-pumps. CAP1 from yeast microsomes bound CaM, and the CAP1-dependent Ca²⁺ transport in yeast was stimulated by CaM. Peptides from the C terminus of CAP1 bound CaM. Anti-CAP1 antibodies specifically recognized a maize microsomal polypeptide that also bound CaM. A similar polypeptide also formed a Ca²⁺-dependent phosphoenzyme. Our results suggest that cap1 encodes a novel form of CaM-regulated Ca²⁺-ATPase in maize. CAP1 appears to be encoded by one or two genes in maize. CAP1 RNA is induced only during early anoxia, indicating that the Ca²⁺-pump may play an important role in O₂-deprived maize cells.