= Claudin-2
<Claudin-2> is a <tight junction> <membrane protein> that can create a pathway selective for small positively charged <ions> between epithelial <cells>. Its presence can make a junction more conductive without creating an indiscriminate hole through the epithelial sheet.
Researchers expressed <claudin-2> in an epithelial layer that initially had low conductance between <cells>. Conductivity increased, and comparison of different salts showed preferential passage of <cations> over <anions>.<Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells by Salah Amasheh and others>[2002, abstract: transfected monolayers and relative ion permeabilities]{ref} This is a useful counterexample to treating every <claudin> as a simple seal: changing the molecular composition of a <tight junction> can change both how much passes and which substances pass.
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