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EMAP, an echinoderm microtubule-associated protein found in microtubule-ribosome complexes
K A Suprenant1, K Dean, J McKee
1Department of Physiology and Cell Biology, University of Kansas, Lawrence, Kansas 66045, USA.
Abstract:
The major non-tubulin polypeptide found associated with microtubules purified from unfertilized sea urchin eggs by cycles of pH-dependent assembly has a Mr of 77,000. The 77,000 Mr polypeptide is heat- and acid-labile, and is antigenically distinct from the mammalian brain MAPs, MAP-2 and tau. Affinity-purified antiserum against the 77,000 Mr polypeptide was used to survey a variety of cells and tissues for the presence of antigenically related polypeptides. A cross-reacting polypeptide, ranging in Mr from 72,000 to 80,000, was found in microtubule preparations from a wide variety of echinoderms, including sea urchins, starfish and sand dollars. Indirect immunofluorescence showed that the polypetide was found in interphase as well as mitotic microtubule arrays. No cross-reacting material was detected in microtubules isolated from marine molluscs, mammalian brain or mouse B16 cultured cells. Because the 77,000 Mr MAP is abundant in echinoderms, we have called it EMAP for echinoderm microtubule-associated protein. Although the precise function of the EMAP is not known, our data suggest that the EMAP is involved in the attachment of ribosomes to microtubules. Large numbers of ribosomes are attached to the walls of EMAP-containing microtubules, but not EMAP-deficient microtubules. Removal of the EMAP from the microtubule by salt-extraction results in the release of ribosomes from the microtubule, indicating that the EMAP may form part or all of the long tapered stalk that connects these two organelles.
Insights
Researchers identified a novel microtubule-associated protein, EMAP, in echinoderms. This protein appears to mediate ribosome attachment to microtubules, suggesting a role in protein synthesis regulation.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Microtubules are crucial cytoskeletal components involved in various cellular processes.
- Microtubule-associated proteins (MAPs) regulate microtubule dynamics and interactions.
- Existing MAPs like MAP-2 and tau are well-characterized in mammalian systems.
Purpose of the Study:
- To identify and characterize major non-tubulin polypeptides associated with sea urchin egg microtubules.
- To determine the distribution and potential function of a novel 77,000 Mr MAP.
Main Methods:
- Purification of microtubules from sea urchin eggs via pH-dependent assembly cycles.
- Biochemical characterization of the 77,000 Mr polypeptide (heat/acid lability, antigenic properties).
- Production of affinity-purified antiserum for immunofluorescence and cross-reactivity studies.
Main Results:
- A major 77,000 Mr polypeptide, termed EMAP (echinoderm microtubule-associated protein), was identified.
- EMAP is heat- and acid-labile and antigenically distinct from mammalian MAPs.
- EMAP was found in microtubules from various echinoderms but not mollusks or mammals.
- Indirect immunofluorescence localized EMAP in both interphase and mitotic microtubule arrays.
- EMAP-containing microtubules showed significant ribosome attachment, which was reduced upon EMAP removal.
Conclusions:
- EMAP is a conserved protein in echinoderms, localized to microtubules in diverse cellular contexts.
- The data strongly suggest EMAP's role in mediating ribosome attachment to microtubules.
- EMAP may form the structural link between microtubules and ribosomes, potentially influencing translation.