The HIV A (sor) gene product is essential for virus infectivity

The genome of the human immunodeficiency virus (HIV) contains several open reading frames (ORFs) not present in other viruses. The 'A' gene¹, also known as Q² P'³, ORF-1⁴ or sor ⁵, partially overlaps the pol gene; its protein product has a relative molecular mass of 23,000 (M_r 23K) and is present in productively infected cells^7-10. The function of this protein is unclear; mutant viruses deleted in 'A' replicate in and kill CD4⁺ lymphocyte lines⁸, but the high degree of conservation of the deduced amino-acid sequence in nine different HIV isolates (80%) and the presence of analogous genes in HIV-2¹¹ and other lentiviruses suggest that the gene function is an important one. Here we describe a mutant virus deficient in the 'A' gene which produces virion particles normally; however, the particles are & sm;1,000 times less infective than wild type. Transcomplementation experiments partially restore infectivity. The mutant virus spreads efficiently when virus-producing cells are co-cultivated with CD4⁺ lymphocytes, however, indicating that HIV can spread from cell to cell in a mechanism that does not require the 'A' gene product and probably does not require the production of infective virus particles.