Characterization of S-AKAP84, a novel developmentally regulated A kinase anchor protein of male germ cells

In mammalian spermatozoa, most of the type IIα isoform of cAMP-dependent protein kinase (PKAIIα) is anchored at the cytoplasmic surface of a specialized array of mitochondria in the flagellar cytoskeleton. This places the catalytic subunits of PKAIIα in proximity with potential target substrates in the cytoskeleton. The mechanism by which PKAIIα is anchored at the outer surface of germ cell mitochondria has not been elucidated. We now report the cloning of a cDNA that encodes a novel, germ cell A kinase anchor protein (AKAP) designated S-AKAP84. S-AKAP84 comprises 593 amine acids and contains a centrally located domain that avidly binds regulatory subunits (RIIα and RIIβ) of PKAIIα and PKAIIβ. The 3.2-kilobase S-AKAP84 mRNA and the cognate S-AKAP84 RII binding protein are expressed principally in the male germ cell lineage. Expression of S-AKAP84 is tightly regulated during development. The protein accumulates as spermatids undergo nuclear condensation and tail elongation. The timing of S-AKAP84 expression is correlated with the de nero accumulation of RIIα and RIIβ subunits and the migration of mitochondria from the cytoplasm (round spermatids) to the cytoskeleton (midpiece in elongating spermatids). Residues 1-30 at the NH₂ terminus of S-AKAP84 constitute a putative signal/anchor sequence that may target the protein to the outer mitochondrial membrane. Immunofluorescence analysis demonstrated that S-AKAP84 is co-localized with mitochondria in the flagellum.