Microtubules are primarily composed of which type of protein?

Microtubules are primarily composed of tubulin, which is a globular protein that assembles into long, hollow cylinders. These microtubules are a key component of the cytoskeleton in eukaryotic cells, providing structural support, maintaining cell shape, and facilitating intracellular transport and cell division. Tubulin exists in two forms, alpha-tubulin and beta-tubulin, which dimerize to form the basic building blocks of microtubules.

In contrast, the other options refer to different proteins that have distinct functions. Actin is a protein that forms microfilaments and is crucial for muscle contraction and cell motility. Collagen is a structural protein found in connective tissues that provides strength and support to tissues. Myosin is a motor protein that interacts with actin to enable muscle contraction and other types of cellular movement. Each of these proteins plays important roles in cellular structure and function, but they are not involved in the formation of microtubules. Thus, the correct identification of tubulin as the primary protein in microtubules highlights its vital role in cellular organization and operation.