CLONING AND SEQUENCE-ANALYSIS OF CANDIDATE HUMAN NATURAL KILLER-ENHANCING FACTOR GENES

A cytosol factor from human red blood cells enhances natural killer (N K) activity. This factor, termed NK-enhancing factor (NKEF), is a prot ein of 44000 M(r) consisting of two subunits of equal size linked by d isulfide bonds. NKEF is expressed in the NK-sensitive erythroleukemic cell line K562. Using an antibody specific for NKEF as a probe for imm unoblot screening, we isolated several clones from a lambda gt11 cDNA library of K562. Additional subcloning and sequencing revealed that th e candidate NKEF cDNAs fell into one of two catagories of closely rela ted but non-identical genes, referred to as NKEFA and B. They are 88% identical in amino acid sequence and 71% identical in nucleotide seque nce. Southern blot analysis suggests that there are two to three NKEF family members in the genome. Analysis of predicted amino acid sequenc es indicates that bath NKEF A and B are cytosol proteins with several phosphorylation sites each, but that they have no glycosylation sites. They are significantly homologous to several other proteins from a wi de variety of organisms ranging from prokaryotes to mammals, especiall y with regard to several well-conserved motifs within the amino acid s equences. The biological functions of these proteins in other species are mostly unknown, but some of them were reported to be induced by ox idative stress. Therefore, as well as for immunoregulation of NK activ ity, NKEF may be important for cells in coping with oxidative insults.