Which of the following proteins is primarily involved in communication between cells?

Receptor proteins play a critical role in cellular communication by acting as sensing molecules on the surface of cells. These proteins bind to specific signaling molecules, such as hormones or neurotransmitters, from other cells. This binding triggers a change in the receptor protein's conformation, which then initiates a cascade of biochemical events within the cell, leading to a specific response. This process is essential for various physiological functions, including cell growth, immune responses, and regulating metabolism.

In contrast, transport proteins are mainly responsible for the movement of substances across cellular membranes; structural proteins provide support and shape to cells and tissues; and membrane proteins, while they can include receptors, also encompass a wider range of functions related to transport, enzymatic activity, and cell signaling. Only receptor proteins are directly involved in the specific process of communication between cells through the binding of external signals.