Molecular models of the sperm head-tail coupling apparatus.

A stable connection between the sperm head (nucleus) and tail (flagellum) is crucial for proper fertility. This linkage is mediated by centrioles, or their remnants, at a structure known as the head-tail coupling apparatus (HTCA). Although many proteins have been implicated at the HTCA, the precise molecular linkage that connects the head and tail is poorly understood. This Review proposes three molecular models for the HTCA based on the presence of three key components: nuclear envelope proteins, cytoplasmic proteins and centriole proteins. As it relates to these models, we discuss the current literature that describes the linkage from nuclear envelope proteins to cytosolic and centriole proteins, including a LINC-complex-based linkage, a nuclear pore complex linkage and a direct linkage that bypasses the outer nuclear membrane. Finally, we discuss outstanding questions in the field and how future studies might delineate the complex molecular machinery at the HTCA.