Which protein is primarily responsible for forming gap junctions?

Gap junctions are specialized intercellular connections that facilitate communication between adjacent cells. They are primarily formed by a group of proteins known as connexins. These connexins assemble to create channels that allow the passage of ions and small molecules, enabling electrical and chemical signaling between cells.

Each gap junction consists of two hemichannels or connexons, which are formed by six connexin proteins. When these connexons from neighboring cells align, they form a complete gap junction that permits the direct transfer of signals, supporting synchronized activity in tissues such as the heart and nervous system.

Other proteins listed in the question have distinct functions. For instance, actin plays a critical role in the cytoskeleton, collagen is important in providing structural support in extracellular matrices, and integrins are involved in cell adhesion and signaling pathways. However, none of these proteins are responsible for the formation of gap junctions like connexins are. Therefore, the protein primarily responsible for forming gap junctions is indeed connexin.