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Formation of double-walled microtubules and multilayered tubulin sheets by basic proteins
1Central Institute of Microbiology and Experimental Therapy, Academy of Sciences of the GDR, Jena.
Abstract:
Some basic proteins enable microtubule protein to form special assembly products in vitro, known as double-walled microtubules. Using histones (H1, core histones) as well as the human encephalitogenic protein to induce the formation of double-walled microtubules, we made the following electron microscopic observations: (1) Double-walled microtubules consist of an "inner" microtubule which is covered by electron-dense material, apparently formed from the basic protein, and by a second tubulin wall. (2) The tubulin of the second wall seems to be arranged as protofilaments, surrounding the inner microtubule in a helical or ring-like manner. (3) The surface of double-walled microtubules lacks the projections of microtubule-associated proteins, usually found on microtubules. (4) In the case of protofilament ribbons (incomplete microtubules), H1 binds exclusively to their convex sides that correspond to the surface of microtubules. Zn2+-induced tubulin sheets, consisting in contrast to microtubules of alternately arranged protofilaments, are covered by H1 on both surfaces. Furthermore, multilayered sheet aggregates appeared. The results indicate that the basic proteins used interact only with that protofilament side which represents the microtubule surface. In accordance with this general principle, models on the structure of double-walled microtubules and multilayered tubulin sheets were derived.
Insights
Basic proteins like histones induce the formation of double-walled microtubules in vitro. These structures feature an inner microtubule coated by basic proteins and an outer tubulin wall, revealing specific protein-tubulin interactions.
Area of Science:
- Biochemistry
- Cell Biology
- Structural Biology
Background:
- Microtubules are essential cytoskeletal components involved in various cellular processes.
- Basic proteins can influence microtubule assembly and structure in vitro.
- Understanding protein-microtubule interactions is crucial for cell biology.
Purpose of the Study:
- To investigate the in vitro assembly of double-walled microtubules induced by basic proteins.
- To elucidate the structural characteristics of these novel microtubule formations.
- To determine the specific binding sites of basic proteins on tubulin structures.
Main Methods:
- Electron microscopy was used to observe the structure of in vitro assembled microtubules.
- Histones (H1, core histones) and human encephalitogenic protein were employed as basic proteins.
- Tubulin protofilament ribbons and Zn2+-induced tubulin sheets were analyzed for protein binding.
Main Results:
- Double-walled microtubules comprise an inner microtubule coated by basic proteins and an outer tubulin wall.
- The outer tubulin wall exhibits helical or ring-like arrangements of protofilaments.
- Basic proteins, such as H1, bind specifically to the convex surface of protofilaments, corresponding to the microtubule outer surface.
- Zn2+-induced tubulin sheets bind H1 on both surfaces, and multilayered aggregates were observed.
Conclusions:
- Basic proteins interact selectively with the outer surface of protofilaments, guiding microtubule assembly.
- The findings provide insights into the structural basis of double-walled microtubule formation.
- Models for double-walled microtubules and multilayered tubulin sheets were proposed based on these interactions.