Sequence analysis and chromosomal localization of human cap Z. Conserved residues within the actin-binding domain may link cap Z to gelsolin/severin and profilin protein families

Abstract

From a human retinal cDNA library, we have isolated cDNAs that are homologs for the α2 and β subunits of chicken Cap Z. The derived human α subunit shares 95% amino acid identity with the chicken α2 subunit; the β subunit is 99% identical to the chicken subunit residues 1-243. The remaining portion of the human β subunit (244-272) diverges significantly with only 8 out of 29 C-terminal amino acids conserved between the two species. This lack of conservation is of particular interest because the chicken C terminus contains an actin-binding domain. Cosedimentation assays with F-actin show that human Cap Z binds actin with an affinity equal that of chicken Cap Z. These results point to the eight shared amino acids as critical for actin binding, three of which are regularly spaced leucines. These apolar residues and one outside the region of divergence align well with those residues of the actin-binding α-helix proposed for gelsolin segment 1. The apolar residues as well as three polar amino acids are also conserved in other capping, capping and severing, and monomer-binding proteins. Amino acid substitutions in the chicken β subunit of the two most highly conserved leucines result in significant decreases in F-actin binding activity. The human α2 gene (CAPZA2) has been mapped to chromosome 7 position q31.2-q31.3 and the β gene (CAPZB) to chromosome 1 region p36.1.