Sperm cells are highly specialized for their function in reproduction and have a unique structure among the human cell types. Similarly, the repertoire of proteins that are present in sperm comprise many cell-type specific components. Researchers affiliated with the Human Protein Atlas have started to map proteins in human sperm using immunofluorescence and confocal microscopy to complement and further build on the comprehensive studies of the sperm proteome that so far mainly have been performed using mass spectrometry. This new approach gives a detailed view of the presence and subcellular localization of proteins in sperm, with single-cell resolution.

Human sperm consists of a head region, which mainly carries the paternal genome and proteins involved in fertilization of the egg, and a tail region, which is essential for motility and navigation. For sperm to be able not only to swim, but also to find their way to the egg, they need to be able to carefully sense their environment and modulate their swimming path accordingly. On the route to the egg, they also need to go through multiple carefully coordinated maturation events in preparation for fertilization. Thus, it is not surprising that sperm cells contain many proteins involved in cell signaling. The signaling pathways in sperm are highly intertwined and not fully elucidated, but they generally converge at careful regulation of intracellular HCO3^-, pH_i, cAMP and Ca²⁺, as well as membrane hyperpolarization. These need to be controlled both in time, but also in subcellular space, and thus the cell signaling proteins that mediate the control are likely to be highly compartmentalized at the subcellular level.

One example is the main Ca²⁺-channel in mammalian sperm, called CatSper. This complex is built from four pore-forming subunits and several accessory subunits. Antibody-based staining of the core subunits, such as CATSPER3 (see figure, left panel), indicates that these proteins are mainly localized to the principal piece of the tail, forming a gradient with higher concentration in the proximal part. In addition, the punctate staining pattern suggests that CatSper is concentrated in distinct patches in the plasma membrane, supporting the idea of carefully organization of CatSper into signaling domains along the flagellum (Chung et al, 2014). Furthermore, antibody-based staining of the accessory subunits indicates that these display variable distribution patterns along the flagellum. For example, CATSPERB (see figure, right panel) is mainly found in the midpiece of the tail. The potential structural and functional implications of this variability remain to be explored, but this is one example that demonstrates the complexity of protein localization in sperm, and the advantage of imaging-based mapping of the sperm proteome.