Cell surface tubulin in leukemic cells: molecular structure, surface binding, turnover, cell cycle expression, and...

Prior research has shown that tubulin is primarily an intracellular protein involved in microtubule formation. However, the presence of tubulin on the cell surface has been a subject of limited investigation. Established knowledge suggests that surface proteins are typically stable and functionally distinct from cytoplasmic proteins. No prior work had resolved whether tubulin could be expressed on the surface of leukemic cells. This gap motivated the exploration of surface tubulin in CEM cells, a human leukemia cell line. The study aimed to determine whether surface tubulin is unique in structure and function compared to intracellular tubulin. The question remained whether surface tubulin binds specifically to membranes and if it is produced by the cell itself. The lack of clarity on the origin and turnover of surface tubulin created a need for detailed molecular and functional analysis.

The study aimed to investigate the molecular structure, binding properties, and origin of cell surface tubulin in CEM cells. The specific problem addressed was whether surface tubulin is distinct from intracellular tubulin and how it is maintained on the cell membrane. The motivation stemmed from the observation that CEM cells express surface tubulin that is iodinatable and rapidly turnover. The researchers sought to determine if surface tubulin is produced by the cell or acquired from the extracellular environment. They also aimed to identify the binding specificity of surface tubulin to different cell types. The study focused on the structural uniqueness of surface tubulin compared to total tubulin. The goal was to clarify the role of a putative receptor-like component in surface tubulin binding. The findings were expected to contribute to understanding the functional relevance of surface tubulin in leukemic cells.

The researchers used iodination to label surface proteins in living CEM cells and analyzed the turnover rate of surface tubulin. They employed trypsin to remove surface tubulin and monitored its return to the cell surface. Peptide mapping was performed on iodinated surface tubulin to compare its primary structure with total CEM and rat brain tubulin. A subfraction of CEM tubulin was isolated from metabolically labeled lysates and tested for specific binding to CEM cells. The binding was assessed for saturation and inhibition using unlabeled supernatants and tubulin from other sources. Normal rat kidney cells and circulating white blood cells were tested for tubulin binding to CEM cells. Activated lymphocytes were analyzed for their ability to produce tubulin that binds to CEM cells. Media from cultured CEM cells was examined for the presence of tubulin to determine its extracellular origin.

Surface tubulin in CEM cells was iodinatable and exhibited a turnover rate identical to other surface proteins. Trypsin removal of surface tubulin was followed by rapid re-expression on the cell surface. Peptide mapping showed that surface tubulin had a similar but not identical structure to total CEM and rat brain tubulin. CEM cells specifically bound a subfraction of CEM tubulin from labeled supernatants, and this binding was saturable. Unlabeled CEM supernatants inhibited binding, but not tubulin from other species. Surface tubulin was more basic than the total tubulin pool. Normal rat kidney cells bound surface tubulin from CEM cells, but normal white blood cells did not. Activated lymphocytes produced tubulin that bound to CEM cells. CEM tubulin was detected in the media of 6-hour cultures, indicating extracellular origin.

The authors concluded that CEM cells produce a unique tubulin that binds to membranes, including those of nontransformed cells. Surface tubulin is iodinatable and rapidly turnover, suggesting active regulation. The binding is specific to CEM-derived tubulin and not to tubulin from other species. The presence of surface tubulin requires both unique tubulin production and a receptor-like binding component. The extracellular origin of surface tubulin was confirmed by its detection in culture media. The binding to normal rat kidney cells indicates a broader interaction potential. The absence of binding to normal white blood cells suggests specificity in the interaction. The study highlights the need for further investigation into the functional role of surface tubulin in leukemic cells.