Nuclear Import of the HIV-1 Core Precedes Reverse Transcription and Uncoating

HIV-1 particles contain a core formed by ~1500 capsid protein monomers housing viral RNA. HIV-1 core uncoating\---|disassembly\---|is required for infection. HIV-1 reverse transcription (RT) occurs before or during uncoating, but the cellular compartment where RT and uncoating occurs is unknown. Using imaging and biochemical assays to track HIV-1 capsids in nuclei during infection, we demonstrated that higher-order capsid complexes or complete cores containing viral genome are imported into nuclear compartments. Additionally, inhibition of RT that stabilizes the core during infection does not prevent capsid nuclear import; thus, RT may occur in nuclear compartments. We separated infected cells into cytosolic and nuclear fractions to measure RT during infection. Most observable RT intermediates were enriched in nuclear fractions, suggesting that most HIV-1 RT occurs in the nuclear compartment alongside uncoating. Thus, nuclear import precedes RT and uncoating, fundamentally changing our understanding of HIV-1 infection.