Читать книгу Introduction to the Human Cell - Danton PhD O'Day - Страница 15

In most cells, the membrane proteins are not randomly localized but exist in complexes that are localized to specific domains. This fits well into our idea that the cell membrane is a fluid “mosaic” model. One of the first cell types where protein membrane domains were identified was the sperm cell as shown in the next figure (Figure 2.9). Sperm components were injected into rabbits to induce antibody formation. Three rabbit antibodies that were produced identified three different regions of the surface (i.e., proteins in the membrane) of the sperm. Before we look at these results in detail, let’s look at the structure of the human sperm cell.

Each of the components of the sperm has a specific function. Their overall job is to get their payload, the haploid male nucleus, into the egg cytoplasm where it can fuse with the female pronucleus to form the diploid, zygote nucleus.

Figure 2.9. The structure of the human sperm cell.

Thus it is clear that each region of the sperm has a specific function. That said, it would also be expected that each of the regions has specific, unique proteins associated with it. However, conjecture in science is of no value without experimentation. So the following experiments were critical to validating these ideas or, if we can be more bold, hypotheses.

Figure 2.10. Fractions of the human sperm cell were injected into rabbits to make polyclonal antibodies which were then used to stain sperm. This revealed that different proteins were localized in different regions of the sperm cell.

This work showed:

•Some proteins are restricted to certain locations

•Each region (acrosome, nucleus, tail) has specific proteins associated with it

•These proteins are not found in the other regions (i.e., they are region-specific)

•It's common for specific regions to serve special functions (e.g. sperm's receptor for egg binding and fusion; junctional adhesion molecules)

The way specific proteins localize and also form complex protein domains is an exciting area of cell biology that is growing rapidly. For example, RACK1 (Receptor for Activated C Kinase 1) organizes many constituents involved in cellular signaling as covered in a future chapter.