I have a question about the length of typical alpha-helix transmembrane domain of membrane proteins. It is known that normally transmembrane alpha-helixes consist of 20-25 aminoacids and have the length about 3,5 nm to perforate the membrane. But what happens if the transmembrane domain is longer than 25 aa? I can't find any information about this problem, maybe here I'll find someone to help me?

Gopla, I am not sure what your question is. Alpha helices, beta barrels, and some more exotic transmembrane elements are evolutionarily designed to fit exactly into a membrane. These structures display hydrophobic domains on their exterior so that they can associate with the hydrophobic center of the membrane, and this is what keeps them in place (and how a membrane is made by amphipathic components for that matter). If an alpha helix were too long some portion of the hydrophobic region would be exposed on the outside or inside of the membrane and this would cause problems. Under normal circumstances the genes program for the exact amino acid sequence required to do the job. SM