TO BEAD OR NOT TO BEAD - LENS-SPECIFIC INTERMEDIATE FILAMENTS REVISITED

For nearly three decades cytoplasmic intermediate filaments (IFs) have been described as 10 nm thick, unbranched ropes radiating from the ce ll nucleus and extending to the plasma membrane. This stereotype is no w being challenged by the discovery and molecular characterization of the beaded filaments (BFs), a novel class of IFs composed of the lens- specific proteins filensin and phakinin, In contrast to 'mainstream' I Fs, BFs have a distinctly nodular appearance and form a meshwork under neath the plasma membrane of the lens fiber cells, In vitro assembly s tudies, expression of filensin and phakinin in cultured cells, and ana lysis of the corresponding genes reveal that these proteins have evolv ed from two different subfamilies of IF proteins, thus yielding a uniq ue structure, The new information provides a basis for understanding h ow the various forms of tissue-specific IF proteins might have develop ed adopting to the constraints of a specialized environment.