The Polymerization of Actin: Its Role in the Generation of the Acrosomal Process of Certain Echinoderm Sperm

Abstract

When Asterias or Thyone sperm come in contact with egg jelly, a long process which in Thyone measures up to 90 µm in length is formed from the acrosomal region . This process can be generated in less than 30 s . Within this process is a bundle of microfilaments . Water extracts prepared from acetone powders of Asterias sperm contain a protein which binds rabbit skeletal muscle myosin forming a complex whose viscosity is reduced by ATP . Within this extract is a protein with the same molecular weight as muscle actin . It can be purified either by collecting the pellet produced after the addition of Mg++ or by reextracting an acetone powder of actomyosin prepared by the addition of highly purified muscle myosin to the extract . The sperm actin can be polymerized and by electron microscopy the polymer is indistinguishable from muscle F-actin . The sperm actin was shown to be localized in the microfilaments in the acrosomal processes by : (a) heavy meromyosin binding in situ, (b) sodium dodecyl sulfate (SDS) gel electrophoresis of the isolated acrosomal processes and a comparison to gels of flagella which contain no band corresponding to the molecular weight of actin, and (c) SDS gel electrophoresis of the extract from isolated acrosomal caps . Since the precursor for the microfilaments in the unreacted sperm appears amorphous, we suspected that the force for the generation of the acrosomal process is brought about by the polymerization of the sperm actin . This supposition was confirmed, for when unreacted sperm were lysed with the detergent Triton X-100 and the state of the actin in the sperm extract was analyzed by centrifugation, we determined that at least 80% of the actin in the unreacted sperm was in the monomeric state .

Localization of [alpha]-actinin and tropomyosin in muscle and nonmuscle systems

“…Fine structure of the acrosomal process Several papers (Tilney, 1975;Tilney _et al, 1973) have given detailed morphological descriptions of echinoderm sperm. Because the studies described in this thesis involve only the isolated acrosomal processes of Limulus sperm, only the fine structure of this process will be discussed here.…”

The polymerization of actin: II. how nonfilamentous actin becomes nonrandomly distributed in sperm: evidence for the association of this actin with membranes

“…The morphology of echinoderm sperm has been previously described (41); it is unnecessary to give more than a brief description here. Lying in an indentation in the nucleus is the spherical acrosoreal vacuole; beneath and also lateral to it is the periacrosomal material (Fig.…”

“…Lying in an indentation in the nucleus is the spherical acrosoreal vacuole; beneath and also lateral to it is the periacrosomal material (Fig. 3) which we know is composed largely of actin (41,37). The periacrosomal material appears finely granular.…”

“…With this process the sperm pierces the jelly of the egg, the membrane at its tip then being able to fuse with the plasma membrane limiting the egg. Tiiney et al (41) demonstrated that the newly formed acrosomal process contains actin filaments. Before induction, no actin filaments could be found in the sperm.…”

“…In order to obtain more information on how the actin remains nonfilamentous, we chose as our model system the acrosomal reaction of certain echinoderm sperm (41). Echinoderm sperm, like the sperm of many aquatic species, must penetrate the protective coats which surround the egg.…”