Calmodulin and HIV type 1: Interactions with gag and gag products

The level of calmodulin increases in cells expressing HIV-1 envelope glycop rotein. Although a calmodulin increase is bound to alter many cellular meta bolic and signaling pathways, the benefits to the virus of these alteration s must be indirect. However, the possibility exists that increased cellular calmodulin benefits the virus by directly associating with nonenvelope vir al proteins. We have, therefore, investigated whether calmodulin can intera ct with HIV structural proteins Gag, p17, and p24. Calmodulin binds Gag and p17 but not p24 in I-125-labeled calmodulin overlays of SDS-polyacrylamide gels. Removal of calcium by addition of EGTA eliminates this binding. A co mputer algorithm for predicting helical regions that should bind calmodulin predicts that there are two calmodulin-binding regions near the N terminus of p17. Intrinsic tryptophan fluorimetry shows that two peptides, each of which includes one of the predicted regions, bind calmodulin: p17(11-25) bi nds calmodulin with a 2-to-1 stoichiometry and dissociation constant of app roximately 10(-9) M-2, and p17(31-46) also binds calmodulin with a dissocia tion constant of about 10(-9) M. These binding sites are nearly contiguous, forming an extended calmodulin-binding domain p17(11-46). In H-9 cells, Ga g and calmodulin colocalize within the resolution of confocal light microsc opy.