Strict conservation of the retroviral nucleocapsid protein zinc finger is strongly influenced by its role in viral infection processes: Characterization of HIV-1 particles containing mutant nucleocapsid zinc-coordinating sequences

The retroviral nucleocapsid (NC) protein contains highly conserved amino ac id sequences (-Cys-X-2-Cys-X-4-His-X-4-Cys-) designated retroviral (CCHC) Z n2+ fingers. The NC protein of murine leukemia viruses contains one NC Zn2 finger and mutants that were competent in metal binding (CCCC and CCHH) pa ckaged wild-type levels of full-length viral RNA but were not infectious. T hese studies were extended to human immunodeficiency virus type 1 (HIV-1), a virus with two NC Zn2+ fingers. Viruses with combinations of CCHC, CCCC, and CCHH Zn2+ fingers in each position of HIV-I NC were characterized. Muta nt particles contained the normal complement of processed viral proteins. F our mutants packaged roughly wild-type levels of genomic RNA, whereas the r emaining mutants packaged reduced levels. Virions with mutated C-terminal p osition NC fingers were replication competent One interesting mutant, conta ining a CCCC Zn2+ finger in the N-terminal position of NC, packaged wild-ty pe levels of viral RNA and showed similar to 5% wild-type levels of infecti vity when examined in CD4-expressing HeLa cells containing an HIV-1 LTR/bet a-galactosidase construct. However, this particular mutant was replication defective in H9 cells; all other mutants were replication defective over th e 8-week course of the assay. Two long terminal repeat viral DNA species co uld be detected in the CCCC mutant but not in any of the other replication- defective mutants. These studies show that the N-terminal Zn2+ finger posit ion is more sensitive to alterations than the C-terminal position with resp ect to replication. Additionally, the retroviral (CCHC) NC Zn2+ finger is r equired for early infection processes. The evolutionary pressure to maintai n CCHC NC Zn2+ fingers depends mainly on its function in infection processe s, in addition to its function in genome packaging.