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Abstract
In celebration of MBoC's first 20 years, members of the Editorial Board, members of
the ASCB Council, and others comment on their favorite MBoC papers from the past two
decades.
This study examines the attributes of keratin intermediate filaments in single, live,
nonmotile epithelial cells under steady-state conditions. From their substantive findings,
the authors proposed a spatially and temporally defined “keratin cycle” that accounts
for the genesis, maturation, and turnover of keratin filaments in the setting of live
epithelial cells in culture. I suspect that, just like me, many researchers did not
expect that the process of assembly to turnover of keratin filaments would be so vectorial
and spatially well-defined, even if it ultimately applies only to a cell culture setting.
The “keratin cycle” also provided a conceptual framework from which to examine the
relationships among keratin filaments, F-actin, microtubules, and adhesion complexes.
The model has been refined since, and several of the predictions it originally generated
have been upheld through subsequent experimentation. Finally, this study provided
an alternative view of intermediate filament dynamics at steady state, complementing
similar efforts from other researchers.
It is generally assumed that turnover of the keratin filament system occurs by exchange of subunits along its entire length throughout the cytoplasm. We now present evidence that a circumscribed submembranous compartment is actually the main site for network replenishment. This conclusion is based on the following observations in living cells synthesizing fluorescent keratin polypeptides: 1) Small keratin granules originate in close proximity to the plasma membrane and move toward the cell center in a continuous motion while elongating into flexible rod-like fragments that fuse with each other and integrate into the peripheral KF network. 2) Recurrence of fluorescence after photobleaching is first seen in the cell periphery where keratin filaments are born that translocate subsequently as part of the network toward the cell center. 3) Partial keratin network reformation after orthovanadate-induced disruption is restricted to a distinct peripheral zone in which either keratin granules or keratin filaments are transiently formed. These findings extend earlier investigations of mitotic cells in which de novo keratin network formation was shown to originate from the cell cortex. Taken together, our results demonstrate that the keratin filament system is not homogeneous but is organized into temporally and spatially distinct subdomains. Furthermore, the cortical localization of the regulatory cues for keratin filament turnover provides an ideal way to adjust the epithelial cytoskeleton to dynamic cellular processes.
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