Recent studies with patients suffering from epidermolysis bullosa simplex associated with muscular dystrophy and the targeted gene disruption in mice suggested that plectin, a versatile cytoskeletal linker and intermediate filament-binding protein, may play an essential role in hemidesmosome integrity and stabilization. To define plectin's interactions with hemidesmosomal proteins on the molecular level, we studied its interaction with the uniquely long cytoplasmic tail domain of the β₄ subunit of the basement membrane laminin receptor integrin α₆β₄ that has been implicated in connecting the transmembrane integrin complex with hemidesmosome-anchored cytokeratin filaments. In vitro binding and in vivo cotransfection assays, using recombinant mutant forms of both proteins, revealed their direct interaction via multiple molecular domains. Furthermore, we show in vitro self-interaction of integrin β₄ cytoplasmic domains, as well as disruption of intermediate filament network arrays and dislocation of hemidesmosome-associated endogenous plectin upon ectopic overexpression of this domain in PtK2 and/or 804G cells. The close association of plectin molecules with hemidesmosomal structures and their apparent random orientation was indicated by gold immunoelectron microscopy using domain-specific antibodies. Our data support a model in which plectin stabilizes hemidesmosomes, via directly interlinking integrin β₄ subunits and cytokeratin filaments.

The yeast open reading frames YOL027 and YPR125 and their orthologs in various eukaryotes encode proteins with a single predicted trans-membrane domain ranging in molecular mass from 45 to 85 kDa. Hemizygous deletion of their human homolog LETM1 is likely to contribute to the Wolf-Hirschhorn syndrome phenotype. We show here that in yeast and human cells, these genes encode integral proteins of the inner mitochondrial membrane. Deletion of the yeast YOL027 gene (yol027Δ mutation) results in mitochondrial dysfunction. This mutant phenotype is complemented by the expression of the human LETM1 gene in yeast, indicating a functional conservation of LetM1/Yol027 proteins from yeast to man. Mutant yol027Δ mitochondria have increased cation contents, particularly K⁺ and low-membrane-potential Δψ. They are massively swollen in situ and refractory to potassium acetate-induced swelling in vitro, which is indicative of a defect in K⁺/H⁺ exchange activity. Thus, YOL027/LETM1 are the first genes shown to encode factors involved in both K⁺ homeostasis and organelle volume control.

Dysfunction of plectin, a 500-kD cytolinker protein, leads to skin blistering and muscular dystrophy. Using conditional gene targeting in mice, we show that plectin deficiency results in progressive degenerative alterations in striated muscle, including aggregation and partial loss of intermediate filament (IF) networks, detachment of the contractile apparatus from the sarcolemma, profound changes in myofiber costameric cytoarchitecture, and decreased mitochondrial number and function. Analysis of newly generated plectin isoform–specific knockout mouse models revealed that IF aggregates accumulate in distinct cytoplasmic compartments, depending on which isoform is missing. Our data show that two major plectin isoforms expressed in muscle, plectin 1d and 1f, integrate fibers by specifically targeting and linking desmin IFs to Z-disks and costameres, whereas plectin 1b establishes a linkage to mitochondria. Furthermore, disruption of Z-disk and costamere linkages leads to the pathological condition of epidermolysis bullosa with muscular dystrophy. Our findings establish plectin as the major organizer of desmin IFs in myofibers and provide new insights into plectin- and desmin-related muscular dystrophies.

Plectin is a major intermediate filament (IF)–based cytolinker protein that stabilizes cells and tissues mechanically, regulates actin filament dynamics, and serves as a scaffolding platform for signaling molecules. In this study, we show that plectin deficiency is a cause of aberrant keratin cytoskeleton organization caused by a lack of orthogonal IF cross-linking. Keratin networks in plectin-deficient cells were more susceptible to osmotic shock–induced retraction from peripheral areas, and their okadaic acid–induced disruption (paralleled by stress-activated MAP kinase p38 activation) proceeded faster. Basal activities of the MAP kinase Erk1/2 and of the membrane-associated upstream protein kinases c-Src and PKCδ were significantly elevated, and increased migration rates, as assessed by in vitro wound-closure assays and time-lapse microscopy, were observed. Forced expression of RACK1, which is the plectin-binding receptor protein for activated PKCδ, in wild-type keratinocytes elevated their migration potential close to that of plectin-null cells. These data establish a link between cytolinker-controlled cytoarchitecture/scaffolding functions of keratin IFs and specific MAP kinase cascades mediating distinct cellular responses.

The enterobacterial pathogen Salmonella induces phagocyte apoptosis in vitro and in vivo. These bacteria use a specialized type III secretion system to export a virulence factor, SipB, which directly activates the host's apoptotic machinery by targeting caspase-1. Caspase-1 is not involved in most apoptotic processes but plays a major role in cytokine maturation. We show that caspase-1–deficient macrophages undergo apoptosis within 4–6 h of infection with invasive bacteria. This process requires SipB, implying that this protein can initiate the apoptotic machinery by regulating components distinct from caspase-1. Invasive Salmonella typhimurium targets caspase-2 simultaneously with, but independently of, caspase-1. Besides caspase-2, the caspase-1–independent pathway involves the activation of caspase-3, -6, and -8 and the release of cytochrome c from mitochondria, none of which occurs during caspase-1–dependent apoptosis. By using caspase-2 knockout macrophages and chemical inhibition, we establish a role for caspase-2 in both caspase-1–dependent and –independent apoptosis. Particularly, activation of caspase-1 during fast Salmonella-induced apoptosis partially relies on caspase-2. The ability of Salmonella to induce caspase-1–independent macrophage apoptosis may play a role in situations in which activation of this protease is either prevented or uncoupled from the induction of apoptosis.