Permeases general amino acid

General amino acid permease (Gapl) Springael and Andr6,1998... 

GAP1 General amino acid permease localization to the plasma membrane is regulated by nitrogen source... 

The penetration of amino acids and sugars into the yeast activates membrane transport systems called permeases. The general amino acid... 

A general amino acid permease (GAP) transports all of the amino acids. The ammonium ion inhibits and represses the GAP. The GAP therefore only appears to be active during the second half of fermentation, when the must no longer contains ammonium. It acts as a nitrogen scavenger towards amino acids (Cartwright et al, 1989). 

The regulation of NCR-sensitive amino acid transporters in Saccharomyces cerevisiae has many points in common with that of catabolic enzymes. Amino acid permeases, as well as some other transporters of nitrogenous nutrients, are integrated into the regulatory circuits, both general and specific, which control catabolic processes. 

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

The basis for this orderly sequence of events appears to involve the nature and specificities of amino acid permeases. Thus, two general permeases may be involved, one showing specificity for group A and group C amino acids, since members of the former group compete with those of the latter [37], and one transporting group B amino acids. 

Figure 1.3. Diagram of the proteolytic systems of lactic acid bacteria, (a) Extracellular components PrtP, cell-envelope proteinase PrtM, proteinase maduration protein Opp, oligopetide permease DtpT, the ion linked trasnsporter for di-and tripeptides and Opt, the ABC transporter for peptides, (b) Intracelullar components pool of about 20-25 peptidases, including general (PepN, PepC) and specific (PepX, PepQ) peptidases, and amino acid catabolic enzymes (carboxylases, aminotransferases, etc.).

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