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Testis- and sperm-specific protein phosphatase, PP1γ2, is a key enzyme regulating sperm function. Its activity decreases during sperm maturation in the epididymis. Inhibition of PP1γ2 leads to motility initiation and stimulation. Our laboratory is focused on identifying mechanisms responsible for the decline in PP1γ2 activity during sperm motility initiation in the epididymis. Previously, using immuno-affinity chromatography, we showed that a mammalian homologue of yeast sds22 is bound to PP1γ2 in motile caudal spermatozoa (Huang Z, et al. Biol Reprod 2002; 67:1936–1942). The objectives of this study were to determine: 1) stoichiometry of PP1γ2-sds22 binding and 2) whether PP1γ2 in immotile caput epididymal spermatozoa is bound to sds22. The enzyme from caudal and caput sperm extracts was purified by column chromatography. Immunoreactive PP1γ2 and sds22 from both caudal and caput …