Identification and characterization of a novel mammalian intermediate filament-associated protein

Naringin-sensitive phosphorylation of plectin, a cytoskeletal cross-linking protein, in isolated rat hepatocytes

To identify phosphoproteins that might play a role in naringin-sensitive hepatocellular cytoskeletal disruption and apoptosis induced by algal toxins, hepatocyte extracts were separated by gel electrophoresis and immunostained with a phosphothreonine-directed antibody. Use of dilute (5%) polyacrylamide gels containing 6 m urea allowed the resolution of one very large (approximately 500-kDa) okadaic acid- and naringin-sensitive phosphoprotein, identified by tryptic fingerprinting, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and immunostaining as the cytolinker protein, plectin. The naringin-sensitive phosphorylation induced by okadaic acid and microcystin-LR probably reflected inhibition of a type 2A protein phosphatase, whereas the naringin-resistant phosphorylation induced by calyculin A, tautomycin, and cantharidin probably involved a type 1 phosphatase. Okadaic acid caused a collapse of the plectin-immunostaining bile canalicular sheaths and the general cytoskeletal plectin network into numerous medium-sized plectin aggregates. Inhibitors of protein kinase C, cAMP-dependent protein kinase, or Ca(2+)/calmodulin-dependent kinase II had moderate or no protective effects on plectin network disruption, whereas naringin offered 86% protection. Okadaic acid induced a naringin-sensitive phosphorylation of AMP-activated protein kinase (AMPK), the stress-activated protein kinases SEK1 and JNK, and S6 kinase. The AMPK-activating kinase (AMPKK) is likely to be the target of inhibition by naringin, the other kinases serving as downstream components of an AMPKK-initiated signaling pathway.

Monoclonal antibody P-31 recognizes a novel intermediate filament-associated protein (p250) in rat podocytes

The visceral glomerular epithelial cells (GECs) or podocytes of the renal glomerulus constitute a highly specialized epithelium. To study the nature of podocytes, we established mouse monoclonal antibodies against GEC. Clone P-31 reacted exclusively with the cytoplasm of GEC by immunofluorescence. Immunoblot analysis with P-31 showed that a single band of 250 kDa was detectable in a glomerular lysate. The 250-kDa polypeptide (p250) was recovered from Triton X-100-insoluble fractions of isolated glomeruli, suggesting that this molecule is associated with the cytoskeleton. Immunogold staining with P-31 demonstrated that the gold particles were located at the intersections of vimentin-type intermediate filaments of podocytes. In developing kidney, this protein first appeared in immature GECs during the S-shaped body stage. In puromycin aminonucleoside nephrosis, p250 was dramatically increased in glomeruli where enhanced desmin expression was observed in GECs. These results indicate that p250 is a novel intermediate filament-associated protein and plays a role in the organization of the intermediate filament network in both normal and diseased conditions.

Immunocytochemical demonstration of a new vimentin-associated protein in 3T3 fibroblasts

Using a xanthophore cytoskeletal preparation as immunogen, we have produced a monoclonal antibody, A2, which recognized a 160 kDa protein in 3T3 fibroblasts. This protein makes up a cytoplasmic filamentous system, which colocalizes with vimentin filaments. When microtubules and actin filaments are dissolved by high salt extraction, staining with antibody A2 is unaffected. Immunoblot analysis confirms that the 160 kDa protein is co-isolated with vimentin during in vivo high salt extraction. Following vinblastine treatment, both the 160 kDa protein and vimentin become localized to perinuclear caps, as do other intermediate filaments and their associated proteins; after vinblastine removal, the immunostaining produced by A2 becomes filamentous. Immunoelectron microscopy demonstrates that antibody A2 stains a filament system with a diameter of about 10 nm. Our observations suggest that the 160 kDa protein may be a new vimentin-associated protein which differs from the intermediate filament-associated proteins previously reported, and is widely distributed in several cell types.

Role of tropomyosin, alpha-actinin, and actin binding protein 280 in stabilizing Triton insoluble F-actin in basal and chemotactic factor activated neutrophils

F-actin is a major component of the neutrophil (PMN) cytoskeleton. In basal PMNs, F-actin exists in two structurally and functionally distinct pools: Triton insoluble F-actin (TIF)--cold insensitive, not depolymerizable by dilution, and distributed in pseudopods and submembranous locations; and Triton soluble F-actin (TSF)--unstable in cold, diffusely distributed, and gelsolin enriched. The element(s) conferring these unique properties to the Triton insoluble F-actin pool are unknown, but logically include distinct actin regulatory proteins. To study the morphologic and functional determinants of the Triton insoluble F-actin pool, the distribution and quantity of three candidate regulatory proteins, alpha-actinin, tropomyosin (TM), and actin binding protein (ABP-280), were compared in F-actin (Triton insoluble and Triton soluble) and G-actin pools isolated from basal and chemotactic factor activated human PMNs in suspension, using immunoblots and ionic extraction. F-actin content was measured by NBDphallacidin binding and gel scans. The results show that: (1) alpha-actinin, actin binding protein 280, and tropomyosin are localized to TIF and excluded from TSF; (2) TM, alpha-actinin, and ABP 280 are required to stabilize fractions of Triton insoluble F-actin in PMNs; and (3) chemotactic factor activation results in release of a fraction of TM from the Triton insoluble F-actin pool in temporal association with F-actin polymerization in the Triton insoluble F-actin pool. Shifts in ABP 280 or alpha-actinin do not occur. The results suggest that TM, alpha-actinin, and ABP 280 provide structure to TIF and that TM release from TIF is involved in chemotactic factor induced actin polymerization in PMNs.

Rapid effects of EGF on cytoskeletal structures and adhesive properties of highly metastatic rat mammary adenocarcinoma cells

In the highly metastatic rat mammary adenocarcinoma cell clone MTLn3, EGF induced increased adhesion to fibronectin while in the human epidermoid carcinoma cell line A431 EGF induced diminished adhesive properties. Flattening of cells with extensive formation of filopodia was observed in MTLn3 cells within 5 min of EGF addition, while in A431 cells EGF induced rounding up and only occasional formation of filopodia. Immunofluorescent analysis revealed extension of microtubules (MT) into the filopodia and Western blot analysis demonstrated an EGF-induced 2- to 3-fold increase in the amount of assembled tubulin in MTLn3 but not in A431 cells. In MTLn3, but only marginally in A431 cells, EGF treatment resulted in phosphorylation of a 280 kD cytoskeleton-associated protein, which was rapid and dose-dependent. These results suggest differential signal transduction pathway of cytoskeleton-associated EGFRs in highly metastatic MTLn3 as compared with A431 cells.

Actin-binding protein is a component of bovine erythrocytes

Actin-binding protein (ABP) is a well-characterized polypeptide capable of crosslinking filamentous actin. To date, this polypeptide has been shown to exist in a number of tissues and cultured cell lines. This report shows that by using a panel of three monoclonal antibodies for immunoblotting and immunofluorescence analysis, that ABP is present in bovine erythrocytes. Moreover, the data obtained suggest that this protein is a component of the erythrocyte membrane skeleton. Additionally, bovine erythrocyte ABP is shown to possess both an apparent molecular weight and an isoelectric point identical to that of bovine smooth muscle filamin, implying that these two polypeptides are identical.

Keratohyalin, trichohyalin and keratohyalin-trichohyalin hybrid granules: an overview

Filaggrin and trichohyalin are the two major intermediate filament associated proteins which interact with keratin filaments in the skin. These two proteins initially accumulate in cytoplasmic granules called keratohyalin or trichohyalin granules which provide prominent morphological hallmarks of differentiation in the epidermis and the inner root sheath of hair follicles, respectively. The contents of each of these granules are modified and subsequently released into the cytoplasm of the fully mature cells where they function in the role of aggregating keratin filament bundles. We are beginning to identify several important aspects relative to the unique biological functions of both filaggrin and trichohyalin during the late stages of keratinocyte differentiation. This overview summarizes recent work on these proteins and will also highlight the existence of novel cytoplasmic granules, keratohyalin-trichohyalin hybrid granules, in dorsal tongue epithelia.

Interaction of trichohyalin with intermediate filaments: three immunologically defined stages of trichohyalin maturation

"Trichohyalin" is a 220-kD protein found in trichohyalin granules that are present as major differentiation products in the medulla and inner root sheath cells of human hair follicles. It was unclear whether this protein served as an intermediate filament precursor in the inner root sheath or as an intermediate-filament-associated (matrix) protein. We have produced a panel of monoclonal antibodies (AE15-17) to this protein and used them to trace its fate during inner root sheath differentiation. These studies have allowed us to define three immunologically distinct forms of this trichohyalin protein. They are 1) the AE15-positive form, which is found throughout all trichohyalin granules; 2) the AE16-positive form, which is localized as discrete punctae on the surface of trichohyalin granules; and 3) the AE17-positive, intermediate-filament-bound form, which associates with the inner root sheath filaments with a regular, 400-nm periodicity. From these results we suggest that the 220-kD trichohyalin protein is an intermediate-filament-associated protein that may play a role in the lateral aggregation, precise alignment, and stabilization of inner root sheath filament bundles.