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Cover  The luminal surface of the mammalian bladder epithelium in contact with the urine is covered almost entirely with unique rigid-appearing membrane plaques. These urothelial plaques consist of two-dimensional crystalline arrays of hexagonally arranged 16-nm protein particles that protrude from the lipid bilayer into the luminal space. The cover photograph of an urothelial plaque and its fast fourier transformation (insert) from a quick-freeze deep-etch study (Kachar et al. [1999] J. Mol. Biol. 285:595-608) shows that the head (apical) domain of the uroplakin particle has a "twisted ribbon" structure, which is exposed without an extensive glycocalyx shield. Each particle interacts closely with the head domains of six neighboring particles. This same study showed that the urothelial particles undergo dynamic structural changes and lateral migration and that head-to-head interaction determines the shape and size of the plaques, as they can change their configuration in response to mechanical perturbations. These urothelial particles are composed of four major integral membrane proetins called uroplakins (Wu et al. [1994] J. Biol. Chem. 269:13716-13724). While the specific function of the uroplakins remains to be determined, there is little doubt that this remarkable specialization of the bladder surface plays a key role as an extremely efficient permeablility barrier and supports the continuous adjustments of the apical surface area during changes of bladder volume.Bechara Kachar