Research
- Our Main Objectives
- Introduction to Keratin Genes and Proteins
- Assembly and Organization of Keratin Filaments
- Keratins fulfill Multiple Roles in Skin and other Epithelia
- Keratin Gene Expression in Epidermis: Basic Principles and Disease Associations
- Introduction to Skin Epithelia
- Figure 1 - Introduction to Keratins
- Table 1 - Keratin-based, Inherited Skin Bullous Disease Affecting Primarily the Epidermis
Assembly and Organization of Keratin Filaments
Owing to the presence of long-range heptad repeats in subdomains 1A, 1B, 2A and 2B of the rod domain (See Figure 1), cytoplasmic IF proteins readily form highly stable coiled-coil dimers, 42-44 nm in length, in which participating monomers exhibit a parallel, in-register alignment. Dimers associate along their lateral surfaces, with an antiparallel orientation, to form apolar tetramers. Tetramers interact, end-to-end and along their lateral surfaces, to yield 10-nm filaments. IF proteins have been difficult to crystallize owing to their insolubility, polymerization-prone character, and lack of assembly inhibitors. Accordingly, there is no atomic-level information about the structure of polymerized IFs. Various analyses in vitro and in select tissues in vivo indicate that mature IFs are comprised of several sub-fibrils (See Figure 1). The presence of a 21-22 nm axial repeat in some EM preparations of filaments implies that coiled-coiled dimers are staggered in a consistent fashion within the mature polymer. Given their exposure at the filament surface, end domains are poised to mediate interactions with other filaments, with cellular proteins, and serve as substrate for post-translational modifications that regulate their assembly, organization, and function. The end domains also contribute to 10 nm filament assembly, though in an accessory fashion compared to the central rod domain.
The organization of cytoplasmic IF networks varies according to cell types and biological contexts. In skin epithelial cells, keratin IFs form a dense, pan-cytoplasmic network anchored at desmosomes (cell-cell) and hemidesmosome (cell-matrix) adhesion sites (See Figure 1). The determinants responsible for these attachments are well known. Proteins of the plakin family serve as ideal molecular bridges mediating many of these attachments. Thus desmoplakin, plectin, and BPAG isoforms, to name a few, feature a modular substructure including peculiar globular domains that house the determinants mediating their binding cytoplasmic IFs. Plectin, in particular, help integrate IFs with the F-actin and microtubule networks. Disrupting the function of plakin proteins in vivo yields clinically significant phenotypes that often overlap with genetic deficiencies in relevant IF genes. IFs also interact with organelles including mitochondria, Golgi, endosomes, and lysosomes. There is recent evidence that attachment of cytoplasmic IFs to the nucleus involves their plectin-mediated interaction with nesprin-3, a newly defined protein that spans the nuclear envelope.
We know comparatively little about the mechanisms that account for the organization of keratin IFs “away from membranes”, i.e., in the cytoplasm. In stratified epithelia, where they are highly abundant, keratin IFs tend to form prominent bundles in the cytoplasm (See Figure 1). Two proteins, filaggrin and trichohyalin, are bona fide keratin IF crosslinkers in vivo, but each show a restricted distribution in late-stage differentiating keratinocytes of epidermis and hair follicles, respectively. Thus, the list of keratin filament bundlers is rather “anemic” when considering the armada of cellular proteins known to crosslink F-actin in vivo. Filament crosslinking and cytoplasmic organization are of crucial importance to the structural support function of keratin filaments in vivo, and also of interest to us.
