Required for male fertility (PubMed:38870534). Plays an essential role in maintenance of sperm head-to-tail attachment during late stages of spermiogenesis, via promoting secure attachment of the nuclear envelop to the flagellum coupling apparatus (Probable) (PubMed:38870534). Component of the LMNB1/SUN5/SEPT12 bridge that connects the sperm proximal centriole to the implantation fossa, the bridge functions to prevent detachment of the proximal centriole from the posterior nucleus during spermiogenesis (PubMed:38870534). Required for localization of SEPT12 at the implantation fossa of sperm cells during spermiogenesis (By similarity). {ECO:0000250|UniProtKB:Q9DA32, ECO:0000269|PubMed:38870534, ECO:0000305|PubMed:28945193}. This is the function of SUN5 (Sad1 and UNC84 domain containing 5, Ensembl gene identifier ENSG00000167098).