Heat shock or drug treatments induced a co-collapse of intermediate filaments and associated small heat shock

The small heat shock proteins form a large protein family comprising proteins important in the cellular stress response. Their ability to protect cells against heat shock and their increased expression as part of the heat shock response contributed to their assignment as heat shock proteins. As their molecular mass was usually less than 30, 000 the term ‘small’ was incorporated to distinguish them from the other heat shock proteins. Small heat shock proteins have been implicated in many different cellular processes, such as suppression of protein aggregation, involvement in the dynamics of cytoskeletal proteins, cellular growth, transcription and differentiation. Of the 6 currently known sHSPs in human (de Jong et al., 1998), some are widely expressed among various tissues, whereas others are restricted to only one or two. For instance, αB-crystallin is not only a major lens protein but it is also abundantly expressed in muscle and astrocytes (Iwaki et al., 1990; Kato et al., 1991b) whereas αA-crystallin is almost entirely restricted to the eye lens (Kato et al., 1991a; Srinivasan et al., 1992). These differences in expression patterns may suggest that small heat shock proteins have general as well as specialised functions in cells. Whatever their specific roles might be, control of protein-protein interactions appears to be the lowest common denominator. It is this chaperone-like property, arresting the heat-induced unfolding of proteins, which has been used to search for physiological substrates. Our studies on the lens cytoskeleton led to the discovery that sHSPs could associate both with intermediate filaments and their soluble subunits (Nicholl and Quinlan, 1994). Previous studies had identified a specific association between αB-crystallin and intermediate filaments in certain human pathologies involving 2099 Journal of Cell Science 112, 2099-2112 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS0341