Pharmacological and physiological evidence indicate that protein phosphorylation is involved in the initiation and regulation of flagellar motility; the presence and anchoring of several kinases and phosphatases in the axoneme also suggest a tight control in the regulation of motility (Porter and Sale, 2000). The hyperpolarization of fish sperm plasma membrane that occurs at spawning stimulates adenylyl cyclase (Morisawa et al., 1999) and the resulting increase in cAMP activates protein kinase A (PKA). A portion of PKA activity is tightly bound to the axoneme, possibly by binding to AKAPs (Feliciello et al., 2001), such as those associated with the central pair apparatus (AKAP240) or with the radial spokes (AKAP97) in Chlamydomonas (Porter and Sale, 2000). PKA is also present in the detergent-soluble extracts of spermatozoa in many species including sea urchins, rainbow trout and mammals (Horowitz et al., 1984; Ishuguro et al., 1982; Yokota and Mabuchi, 1990).
Several axonemal proteins, of 45, 130 and 500kDa in sea urchin (Bracho et al., 1998), 21 kDa in salmons (Inaba et al., 1999) and 21 and 26 kDa in Ciona (Nomura et al., 2000) are phosphorylated by PKA when motility is initiated. In chum salmon spermatozoa, one of the seven proteins phosphorylated in relation to motility initiation is a 21 kDa light chain of the outer arm dynein (Inaba et al., 1999). Dynein light chains from many fish and sea urchin species have a PKA-dependent phosphorylation motif and are homologous to t-complex testis expressed 2 (Tctex2). In salmonid fishes, proteasomes localized near the outer dynein arm may regulate the phosphorylation of the dynein light chain and the regulatory subunit of PKA (Inaba et al., 1998). Furthermore, a tyrosine kinase is activated downstream of PKA and is responsible for the phosphorylation of a protein called movement-initiating phosphoprotein (Morisawa, 1994). Itoh et al. (2003) observed that the catalytic subunit of PKA in rainbow trout spermatozoa is anchored to the outer dynein arm of axonemes. The co-localization of PKA and its regulator, proteasomes and its substrate the dynein light chain, suggest the existence of an enzyme-substrate network that allows a rapid phosphorylation of the dynein light chain to support initiation of motility within 1s of exposure to fresh water (Itoh et al., 2003). Other enzymes are involved in flagellar motility, such as calmodulin, a calmodulin-dependent kinase II, casein kinase I, protein phosphatases, but only few of the protein targets of these enzymes are known (Inaba, 2002; Smith, 2002; Smith and Yang, 2004).
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