Advanced Molecular Cell Biology - Lecture 4

In multicellular organisms, selective expression of certain _______ and the molecules involved in _____ ________ allow cells to respond _________ to particular stimuli

receptors, signal transduction, specifically

a channel is a ___________ _______ that transports molecules from one side of the membrane to the other, they are specific and can be open or gated

transmembrane protein

describe the configuration of open/leakage channel:

always open

describe the configuration of voltage gated channels:

opens (transiently) in response to a change in the membrane voltage (membrane potential)

describe the configuration of ligand gated channels:

opens (closes) in response to a specific extracellular/intracellular molecule (depending on concentration gradient)

name the 3 essential functions if ion channels:

• transport ions across membrane (secretion/absorption of fluids)

• regulate membrane potentials (nerve and muscle cells for high-speed communication)

• calcium influx into the cytoplasm (secretion and muscle contraction)

what are the main structural features of all ion channels?

• transmembrane proteins made up of 2 or more alpha-helices that cross the lipid bilayer

• made up of 2 - 6 subunits which usually surround the pore

• ion channels are classified into subgroups based on; gating mechanism and ion selectivity of the pore (defined by physical size of ‘filter’ and amino acids lining the pore)

fill in the blanks:

there are ____ genes in humans code for membrane channels, knowledge of structure reveals evolutionary relationship

400

which channel from the bacterium Streptomyces lividans serves as a model for all channels?

the pH-regulated K+ channel KcsA

describe the molecular structure of a simple ion (K+) channel:

• 4 subunits, 2 transmembrane domains for each subunit, with a pore in the middle of them

• Transmembrane helicase structure forms a p-loop (pore) - highly selective in terms of size and charge

• On the cytoplasmic side, transmembrane domains are more tightly packed creating a ‘gate’

on cytoplasmic side of a simple K+ channel, transmembrane domains are more tightly packed creating a ‘gate’, what are 3 factors controlling this gate?

membrane potential, mechanical stress and ligands

what are the 2 main functions of voltage gated ion channels?

• sodium and potassium ions create action potentials in excitable cells

• Calcium ions transported into the cytoplasm, where a second messenger elicits a cellular response

voltage gated ion channels are based on similar structure to a simple ion channel except;

• additional helices S1-S4 form a separate ‘voltage sensing domain’ lateral to the subunits

• large polypeptides that extend into the cytoplasm

• plugging mechanism