The perinuclear matrix as a structural element of the mouse sperm nucleus

The mouse sperm nucleus, after the removal of protamines and DNA, consisted of a skeletal structure that conformed to the original nuclear shape. Sperm were extracted with 1% SDS, and the isolated nuclei, along with the enveloping perinuclear theca, were incubated in 25 mM dithiothreitol, and exposed to different reagents in an effort to displace the protamines, P1 and P2. Protamines, labeled with [3H]arginine, were displaced from the nucleus by CaCl2.MgCl2, but only partially by anionic detergents, monovalent cations, and polyvalent anions. Displacement of P1 and P2 was achieved by digesting the nuclei with DNase I and simultaneously extracting with CaCl2.MgCl2 (3:2; mol:mol) in stepwise increments of 125, 150, 175, 200, and 250 mM. Protamine displacement was concentration-dependent, occurring with an EC50 of approximately 205 mM and with maximal displacement at approximately 250 mM CaCl2.MgCl2. The nucleus was reduced to a skeletal structure consisting of the perinuclear theca and an internal network of transverse fibers. The evidence was consistent with the former being derived from the perforatorium and postacrosomal nuclear sheath (both cytoplasmic structures), whereas the fibers were most likely of nuclear origin. By SDS-PAGE and isoelectric focusing (IEF), perinuclear matrices consisted of greater than or equal to 230 protein spots, with M(r)s in the range of 70,000 to 8000 and pIs of greater than or equal to 7.5 to approximately 4.7, respectively. Monoclonal antibodies prepared against perinuclear matrices bound to specific proteins on IEF immunoblots and, based on light and electron microscopic observations, to discrete domains of the sperm perinuclear theca and nucleus.