A type of protein embedded in the cytoplasmic membrane of the cell which transmits signals from outside the cell (such as from hormones binding to receptors on the outside of the cell) to the inside of the cell, where it causes some sort of biochemical reaction within the cell to the signal (such as the altering of metabolic pathways or gene expression). The process by which the protein does this is unclear but involves exchanging a molecule of GDP for a molecule of GTP.