FIG. 2.

Mechanisms of signal perception. Generalized signal is depicted as a filled circle. Pathways 1a and 1b involve external perception of the signal either through an external binding protein intermediate that presents the signal to the transmembrane receptor (1a) or through direct interaction of the signal with a transmembrane receptor (1b). Receipt of the external signal is transduced to target processes within the cell. In pathways 2a and 2b, the signal is internalized through passive diffusion (2a) or through specific import mechanisms (2b) and interacts with an intracellular receptor, which in turn modulates cellular processes.