The Cytoplasm

Until the last decade or so, many intracellular signaling pathways were viewed as processes in which second messengers diffused uniformly through a well-mixed milieu of the cell’s cytoplasm to reach their targets. Although it was recognized that the cell’s interior was compartmentalized, this compartmentalization was believed to be largely defined by internal membranes, such as the nuclear envelope, endoplasmic reticulum (ER), sarcoplasmic reticulum (SR), and mitochondria. But like the joke about the person who has lost his keys in the dark but looks for them under the street lamp because the light is better, this view of the cytoplasm as a well-mixed milieu was less of a proven fact than a simplifying assumption. Over the last decade, advances in subcellular imaging have dramatically upset this view, so that now a high degree of compartmentalization of signaling pathways within the cytoplasm is considered the norm rather than the exception. It is now clear that the cytoplasm has a highly organized cytoskeleton and sophisticated molecular trafficking mechanisms that direct and tether proteins into macroaggregates at specific locations to facilitate localized signaling. The cytoplasm is now viewed as a system of microdomains with restricted diffusion (eg, hierarchical Ca2+ signaling) and direct channeling of substrates to enzymes (eg, protein kinases/phosphatases cascades).In the field of metabolism, subcellular compartmentation of energy production has been a well-accepted fact ever since mitochondria were identified as the engines driving aerobic high-energy phosphate production. In addition, glycolytic enzymes complexes are well-known to be associated with specific intracellular structures, such as the SR.1 On the flip side, however, energy consumption by the cell has traditionally been viewed as a fairly democratic process, with high-energy phosphates freely diffusing throughout the cytoplasm to be consumed wherever they are needed. In tissues with high energy requirements, such as muscle, the creatine …