Porin (1PRN) - The nonpolar interior of membranes limits the free diffusion of polar molecules. In most membranes various proteins serve as channels and pumps which regulate the movement of ions, metabolites and even water across the membrane. Due to the specificity of these proteins there is a high degree of control over what passes through the membrane. Several membranes such as the outer membranes of mitochondria, chloroplasts and bacteria, however, do not need the same high degree of control over what gets across. Rather than synthesizing many selective proteins to carry polar molecules across, it appears that cells make only a few types of transporters, like the porin pictured below, which let almost anything below a certain size across. It may simply be more cost effective to put a few porins in these membranes then many more selective proteins. These "leaky" membranes are still effective barriers to the movement of large molecules such as proteins.

Below are two views of a model of porin from the bacterium Rhodopseudomonas blastica. In the left image the model is shown from the side as if you are in the middle of the membrane. The height of the protein is about the same as the thickness of the membrane in which it sits. The right image shows the model from the end where the central "hole" through which ions and metabolites pass is clearly visible. Regions of secondary structure are shown in yellow (beta sheet), pink (alpha helix) and blue (turn). The strands of color follow the backbone of the molecule. If all of the atoms and bonds in the protein were visualized the image would be much too complicated. Why is the tertiary structure of porin described as a beta barrel?

8/22/04 Copyright (C) 2004, Jonathan Monroe, monroejd@jmu.edu. All rights reserved.
URL: http://csm.jmu.edu/biology/courses/bio220/porin.html