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Protein Domains Govern the Intracellular Distribution of Mouse Sperm AKAP4¹

Center for Research on Reproduction and Women's Health, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania 19104

ABSTRACT

A-kinase anchor proteins (AKAPs) spatially restrict cAMP-dependent protein kinase by tethering it to various cellular structures. In the polarized sperm cell, various compartmentalized functions, such as motility generated by the flagellum, are modulated by cAMP-dependent protein kinase. This important regulatory enzyme is associated with AKAP4, the principal component of the fibrous sheath; AKAP4 is synthesized as a precursor, pro-AKAP4, which is cleaved into mature AKAP4 during fibrous sheath assembly. To define the domains responsible for the intracellular distribution and assembly of AKAP4 into a macromolecular complex, various AKAP4-green fluorescent protein (GFP) constructs were introduced into somatic cell lines. The presence of the pro domain, either alone or as part of pro-AKAP4, resulted in a diffuse cytoplasmic localization of the GFP fusion protein, suggesting that, the pro domain keeps the AKAP4 precursor unassembled in vivo until it is transported to the developing tail structure and incorporated into the fibrous sheath. When the mature AKAP4-GFP fusion protein was expressed, it localized in a punctate cytoplasmic pattern. Two domains critical for this punctate localization, T_2a and T_2b, are homologous to the T₂-tethering domain of rat AKAP5 that is important for binding to the actin cytoskeleton in transfected HEK293 cells. In contrast to AKAP5, the distribution of AKAP4 was dependent on the microtubular cytoskeleton. The interaction of AKAP4 with the microtubular network provides evidence that the longitudinal columns of the fibrous sheath, which contain AKAP4, may interact directly with the outer microtubular doublets of the sperm axoneme.

gamete biology, gametogenesis, sperm, sperm motility and transport, spermatid

¹ Supported by NIH HD06274 to S.B.M. and G.L.G.

² Correspondence: Stuart B. Moss, Center for Research on Reproduction and Women's Health, Rm. 1312 BRB II/III, 421 Curie Blvd., University of Pennsylvania Medical Center, Philadelphia, PA 19104-6142. FAX: 215 573 7627; smoss{at}mail.med.upenn.edu

³ Current address: Institute of Molecular Biology, University of Oregon, Eugene, OR 97403.

⁴ Current address: Neotropix, Inc., 351 Phoenixville Pike, Malvern, PA 19355.

⁵ These authors contributed equally to this work.

⁶ These authors were coprincipal investigators of this work.