Importin, which transports target proteins into the nucleus (PubMed:10366588, PubMed:10713112, PubMed:11517331, PubMed:12628928, PubMed:24449914). Specifically mediates the nuclear import of splicing factor serine/arginine (SR) proteins, such as RBM4, SFRS1 and SFRS2, by recognizing phosphorylated SR domains (PubMed:10366588, PubMed:10713112, PubMed:11517331, PubMed:12628928, PubMed:24449914). Also mediates the nuclear import of serine/arginine (SR) protein CPSF6, independently of CPSF6 phosphorylation (PubMed:30916345, PubMed:31465518). The nuclear import process is regulated by the small GTPase Ran that partitions between cytoplasm and nucleus in the predominantly GDP- and GTP-bound form, respectively (PubMed:23878195, PubMed:24449914). Importin associates with target cargo proteins in the cytoplasm, and the competitive binding of GTP-bound Ran induces the release of cargos in the nucleus (PubMed:23878195, PubMed:24449914). (Microbial infection) Involved in immunodeficiency virus (HIV-1) infection by importing the pre-integration complex (PIC) into the nucleus (PubMed:18722123, PubMed:21901095, PubMed:22398280, PubMed:29329553). Required for a nuclear maturation step of HIV-1 prior to integration (PubMed:21901095, PubMed:22398280). Interacts with (GTP-bound) Ran (PubMed:23878195, PubMed:24915079, PubMed:24449914). Interacts with (phosphorylated) SFRS1 and SFRS2; leading to their nuclear import (PubMed:10366588, PubMed:10713112, PubMed:11517331). Interacts with NUP62 (PubMed:11517331, PubMed:24449914). Interacts with RBM4 (PubMed:12628928). Interacts with CPSF6, promoting its nuclear import (PubMed:24449914). (Microbial infection) Interacts with the HIV-1 pre-integration complex (PIC), which is composed of viral genome, matrix protein, Vpr and integrase (PubMed:18722123, PubMed:29329553). Interacts with HIV-1 integrase protein; the interaction is direct (PubMed:29329553). Localizes to the nuclear envelope and annulate lamellae, which consists in stacks of endoplasmic reticulum membranes containing a high density of nuclear pores. Expressed in skeletal muscle. Variations in TNPO3 are associated with resistance or susceptibility to immunodeficiency virus type 1 (resistance or susceptibility to HIV-1) [MIM:609423] (PubMed:31465518). A variation that causes LGMDD2 muscular dystrophy induces protection against HIV-1 infection (PubMed:31465518). The disease is caused by variants affecting the gene represented in this entry.