The Murid Herpesvirus-4 gH/gL Binds to Glycosaminoglycans

The first contact a virus makes with cells is an important determinant of its tropism. Murid Herpesvirus-4 (MuHV-4) is highly dependent on glycosaminoglycans (GAGs) for cell binding. Its first contact is therefore likely to involve a GAG-binding virion glycoprotein. We have previously identified two such proteins, gp70 and gp150. Gp70 binds strongly to GAGs. However, deleting it makes little difference to MuHV-4 cell binding or GAG-dependence. Deleting gp150, by contrast, frees MuHV-4 from GAG dependence. This implies that GAGs normally displace gp150 to allow GAG-independent cell binding. But the gp150 GAG interaction is weak, and so would seem unlikely to make an effective first contact. Since neither gp70 nor gp150 matches the expected profile of a first contact glycoprotein, our understanding of MuHV-4 GAG interactions must be incomplete. Here we relate the seemingly disconnected gp70 and gp150 GAG interactions by showing that the MuHV-4 gH/gL also binds to GAGs. gH/gL-blocking and gp70-blocking antibodies individually had little effect on cell binding, but together were strongly inhibitory. Thus, there was redundancy in GAG binding between gp70 and gH/gL. Gp150-deficient MuHV-4 largely resisted blocks to gp70 and gH/gL binding, consistent with its GAG independence. The failure of wild-type MuHV-4 to do the same argues that gp150 is normally engaged only down-stream of gp70 or gH/gL. MuHV-4 GAG dependence is consequently two-fold: gp70 or gH/gL binding provides virions with a vital first foothold, and gp150 is then engaged to reveal GAG-independent binding

Fibronectin receptors from Staphylococcus aureus

Fibronectin which is recognized for its ability to mediate substrate adhesion of eucaryotic cells has also been shown to bind to Staphylococcus aureus (Kuusela, P. (1978) Nature (Lond)2 76, 718-720). A further characterization of the interaction of fibronectin with staphylococci is presented here. The binding of 1251- fibronectin to S. aureus, strain Cowan 1, is specific, time-dependent, functionally irreversible, and occurs to both live and heat-killed cells. Furthermore, staphylocci may be saturated with fibronectin at a level which suggests the presence of 250 receptors/cell. A lysate produced by digestion of staphylococcal cells with a bacteriolytic enzyme (lysostaphin) inhibited the binding of 125I-fibronectin to bacteria. The lysate was depleted of its inhibitory activity by passage through a column of Sepharose substituted with fibronectin. The inhibitory activity was destroyed when the lysate was incubated with trypsin or pronase, and a lysate prepared from trypsin-treated cells did not have inhibitory activity. These data suggest that the inhibitory activity of the lysate is due to solubilized surface proteins acting as receptors for fibronectin. Staphylococcal mutants that selectively had lost protein A or fibronectin receptors could be isolated, which suggests the presence of fibronectin receptors distinctly different from protein A. Externally 125 I-labeled proteins from the different mutants were analyzed by affinity chromatography on fibronectin-Sepharose followed by gel electrophoresis. The fibronectin receptor was tentatively identified as a protein with an apparenMt , = 18,000. This component was found in fibronectin-binding strains but was absent in strains deficient in fibronectin receptors