Membrane skeletal proteins and their integral membrane protein anchors are targets for tyrosine and threonine kinases in Euglena.

Proteins of the membrane skeleton of Euglena gracilis were extensively phosphorylated in vivo and in vitro after incubation with [32P]-orthophosphate or gamma-[32P] ATP. Endogenous protein threonine/serine activity phosphorylated the major membrane skeletal proteins (articulins) and the putative integral membrane protein (IP39) anchor for articulins. The latter was also the major target for endogenous protein tyrosine kinase activity. A cytoplasmic domain of IP39 was specifically phosphorylated, and removal of this domain with papain eliminated the radiolabeled phosphoamino acids and eliminated or radically shifted the PI of the multiple isoforms of IP39. In gel kinase assays IP39 autophosphorylated and a 25 kDa protein which does not autophosphorylate was identified as a threonine/serine (casein) kinase. Plasma membranes from the membrane skeletal protein complex contained threonine/serine (casein) kinase activity, and cross-linking experiments suggested that IP39 was the likely source for this membrane activity. pH optima, cation requirements and heparin sensitivity of the detergent solubilized membrane activity were determined. Together these results suggest that protein kinases may be important modulators of protein assembly and function of the membrane skeleton of these protistan cells.

Identification of a region of mouse zona pellucida glycoprotein mZP3 that possesses sperm receptor activity.

The ability of mouse zona pellucida glycoprotein ZP3 (mZP3) to function as a sperm receptor is attributable to certain of its oligosaccharides, not to its polypeptide (P. M. Wassarman, 1990. Development 108, 1-17). Here, purified, radioiodinated mZP3 was digested by either papain or V8 protease, and the glycopeptides produced were fractionated by HPLC and assayed for sperm receptor activity in vitro. Each proteolytic digest of mZP3 contained a heavily glycosylated peptide, approximately 55,000 apparent M(r), that exhibited sperm receptor activity in vitro. To determine the region of mZP3 polypeptide from which the active glycopeptides were derived, Western gel immunoblotting, employing an antiserum directed against a specific mZP3 peptide epitope, and automated amino-terminal amino acid sequencing were employed. Results of these experiments strongly suggest that the active glycopeptides produced by digestion of mZP3 with either papain or V8 protease are derived from the same region of the carboxy-terminal half of the mZP3 polypeptide. These and other findings are discussed in terms of mZP3 structure and function.

A 39-kD plasma membrane protein (IP39) is an anchor for the unusual membrane skeleton of Euglena gracilis.

The major integral plasma membrane protein (IP39) of Euglena gracilis was radiolabeled, peptide mapped, and dissected with proteases to identify cytoplasmic domains that bind and anchor proteins of the cell surface. When plasma membranes were radioiodinated and extracted with octyl glucoside, 98% of the extracted label was found in IP39 or the 68- and 110-kD oligomers of IP39. The octyl glucoside extracts were incubated with unlabeled cell surface proteins immobilized on nitrocellulose (overlays). Radiolabel from the membrane extract bound one (80 kD) of the two (80 and 86 kD) major membrane skeletal protein bands. Resolubilization of the bound label yielded a radiolabeled polypeptide identical in Mr to IP39. Intact plasma membranes were also digested with papain before or after radioiodination, thereby producing a cytoplasmically truncated IP39. The octyl glucoside extract of truncated IP39 no longer bound to the 80-kD membrane skeletal protein in the nitrocellulose overlays. EM of intact or trypsin digested plasma membranes incubated with membrane skeletal proteins under stringent conditions similar to those used in the nitrocellulose overlays revealed a partially reformed membrane skeletal layer. Little evidence of a membrane skeletal layer was found, however, when plasma membranes were predigested with papain before reassociation. A candidate 80-kD binding domain of IP39 has been tentatively identified as a peptide fragment that was present after trypsin digestion of plasma membranes, but was absent after papain digestion in two-dimensional peptide maps of IP39. Together, these data suggest that the unique peripheral membrane skeleton of Euglena binds to the plasma membrane through noncovalent interactions between the major 80-kD membrane skeletal protein and a small, papain sensitive cytoplasmic domain of IP39. Other (62, 51, and 25 kD) quantitatively minor peripheral proteins also interact with IP39 on the nitrocellulose overlays, and the possible significance of this binding is discussed.

Properties and topography of the major integral plasma membrane protein of a unicellular organism.

The cellular distribution, membrane orientation, and biochemical properties of the two major NaOH-insoluble (integral) plasma membrane proteins of Euglena are detailed. We present evidence which suggests that these two polypeptides (Mr 68 and 39 kD) are dimer and monomer of the same protein: (a) Antibodies directed against either the 68- or the 39-kD polypeptide bind to both 68- and 39-kD bands in Western blots. (b) Trypsin digests of the 68- and 39-kD polypeptides yield similar peptide fragments. (c) The 68- and 39-kD polypeptides interconvert during successive electrophoresis runs in the presence of SDS and beta-mercaptoethanol. (d) The 39-kD band is the only major integral membrane protein evident after isoelectric focusing in acrylamide gels. The apparent shift from 68 to 39 kD in focusing gels has been duplicated in denaturing SDS gels by adding ampholyte solutions directly to the protein samples. The membrane orientation of the 39-kD protein and its 68-kD dimer has been assessed by radioiodination in situ using intact cells or purified plasma membranes. Putative monomers and dimers are labeled only when the cytoplasmic side of the membrane is exposed. These results together with trypsin digestion data suggest that the 39-kD protein and its dimer have an asymmetric membrane orientation with a substantial cytoplasmic domain but with no detectable extracellular region. Immunolabeling of sectioned cells indicates that the plasma membrane is the only cellular membrane with significant amounts of 39-kD protein. No major 68- or 39-kD polypeptide bands are evident in SDS acrylamide gels or immunoblots of electrophoresed whole flagella or preparations enriched in flagellar membrane vesicles, nor is there a detectable shift in any flagellar polypeptide in the presence of ampholyte solutions. These findings are considered with respect to the well-known internal crystalline organization of the euglenoid plasma membrane and to the potential for these proteins to serve as anchors for membrane skeletal proteins.