What is the structure of a K+ channel?

The structure of a potassium (K+) channel is indeed characterized as a homotetramer forming a cone shape. This means that it consists of four identical subunits, which come together to create a central pore through which potassium ions can pass. Each subunit contributes to the overall structure, which is vital for the channel's function in selectively allowing K+ ions to move across the cell membrane while excluding other ions.

The cone shape formed by the tetrameric structure is significant because it offers a non-linear pathway for the ions, which can help facilitate their movement in response to the electrochemical gradients across the membrane. This configuration is essential for the channel's conductivity and selectivity.

In contrast, options that describe a monomeric structure with a small opening or a dimeric structure with large extracellular domains do not accurately reflect the established structural biology of K+ channels. Additionally, a single polypeptide with no subunits would not be capable of achieving the necessary complexity and function associated with ion channel activities. The tetrameric nature of K+ channels is a crucial feature that underpins their role in cellular excitability and signaling.