Pe. Visconti et al., REGULATION, LOCALIZATION, AND ANCHORING OF PROTEIN-KINASE A SUBUNITS DURING MOUSE SPERM CAPACITATION, Developmental biology, 192(2), 1997, pp. 351-363
The molecular basis of mammalian sperm capacitation, defined as those
biochemical and functional changes that render the sperm competent to
fertilize the egg, is poorly understood. This extratesticular maturati
onal process is accompanied by the activation of a unique signal trans
duction pathway involving the cAMP-dependent up-regulation of protein
tyrosine phosphorylation presumably through the activation of protein
kinase A JFK-A). We demonstrate in this report that capacitation of ca
uda epididymal mouse sperm in vitro was accompanied by a time-dependen
t increase in PK-A activity. This increase in PK-A activity did not oc
cur in a medium that does not support capacitation. While PK-A catalyt
ic and RI/RII regulatory subunits, as well as PK-A enzyme activity, we
re found in both the Triton X-100-soluble and -insoluble fractions of
the sperm, the increase in PK-A activity accompanying capacitation was
associated with enzyme activity found in the soluble fraction. Moreov
er, the regulatory and catalytic subunits of PK-A were observed by ind
irect immunofluorescence to be present throughout the head, midpiece,
and principal piece of the sperm. Thus, PK-A appears to be functional
in multiple compartments of this highly differentiated cell. A fractio
n of the Triton X-100-insoluble PK-A is presumably tethered by AKAP82,
the major protein of the fibrous sheath of the sperm flagellum which
anchors and compartmentalizes PK-A to the cytoskeleton via the RII sub
unit of PK-A. Using various recombinant truncated AKAP82 constructs in
a gel overlay assay, the RII subunit-binding domain of this protein w
as mapped to a 57-amino-acid residue region at its N-terminus. Compute
r analysis revealed a 14-amino-acid region that resembled the RII-bind
ing domains of other A Kinase Anchor Proteins. A synthetic peptide cor
responding to this domain inhibited AXAP82-RII binding in a gel overla
y assay, providing further support that AKAP82 is an anchoring protein
for the subcellular localization of PK-A in the mouse sperm fibrous s
heath. This work, along with previous findings that cAMP is a key inte
rmediary second messenger in regulating protein tyrosine phosphorylati
on and capacitation, further supports the importance of PK-A in these
processes and necessitates a further understanding of the contribution
of both the soluble and insoluble forms of PK-A, as well as AKAP82, t
o sperm function. (C) 1997 Academic Press.