Nectin-2 is a cell adhesion molecule encoded by a member of the poliovirus receptor gene family. This family consists of human, monkey, rat, and murine genes that are members of the immunoglobulin gene superfamily. Nectin-2 is a component of cell-cell adherens junctions and interacts with l-afadin, an F-actin-binding protein.Disruption of both alleles of the murine nectin-2 gene resulted in morphologically aberrant spermatozoa with defects in nuclear and cytoskeletal morphology and mitochondrial localization. Homozygous null males are sterile, while homozygous null females, as well as heterozygous males and females, are fertile. The production by nectin-2 ؊/؊ mice of normal numbers of spermatozoa containing wild-type levels of DNA suggests that Nectin-2 functions at a late stage of germ cell development. Consistent with such a role, Nectin-2 is expressed in the testes only during the later stages of spermatogenesis. The structural defects observed in spermatozoa of nectin-2 ؊/؊ mice suggest a role for this protein in organization and reorganization of the cytoskeleton during spermiogenesis.The human poliovirus receptor (Pvr) is a member of the immunoglobulin superfamily of proteins (38) [7]). Only pvr, agm1, and agm2 encode proteins that can function as cell receptors for poliovirus (18, 23; V. Racaniello, unpublished data, 1999). Pvr, Prr1, Prr2, and Mph/murine Prr2 are entry cofactors for alphaherpesviruses (9, 37).The cellular functions of members of the Pvr protein family are not known. Some members of the Ig superfamily are involved in cell-cell and cell-extracellular matrix interactions, and others are receptors for cytokines and growth factors. Expression of human Prr2 or Mph/murine Prr2 in cultured cells leads to aggregation, suggesting that these proteins are homophilic adhesion molecules (1, 20). The cytoplasmic domains of Prr1 and Prr2 proteins interact with l-afadin, an actin filament-binding protein (35). l-Afadin is ubiquitously expressed but is localized at specialized membrane structures, called adherens junctions, which are involved in cell-cell adhesion (22). l-Afadin contains one PDZ domain through which it interacts with a COOH-terminal amino acid motif of Prr1 and Prr2 as well as an actin filament-binding domain. Thus, Prr1 and Prr2 are linked to the cytoskeleton through l-afadin. The Prr1 and Prr2 proteins have been renamed Nectin-1 and Nectin-2 (35); the new terminology is used in this paper.To provide information on the function of Pvr family members, we disrupted the murine nectin-2 gene. Male mice lacking both alleles of nectin-2 are infertile and produce morphologically aberrant spermatozoa. Heterozygous males and females and homozygous null females are fertile and have no overt developmental defects. The heads of spermatozoa from nectin-2 Ϫ/Ϫ mice contain mitochondria, dense outer filaments, and misshapen nuclei, and the mitochondrial sheath of the middle piece is disorganized. These morphological defects may result from an overall disruption of cytoskeletal structure. In normal mice...