Hi,
While reading mechanisms of microtubule assembly, I came across the following line: 'At least 40 tubulin subunits would be required to close one turn of the three-start helix, and 66 molecules are required for that of five-start helx'. May someone please tell me what does it mean to be three-start helix or five start helix.

Thanks,
Manu

A three-start helix means that the helix spans three subunits of a protofilament before it completes one turn.

A five-start helix means that the helix spans five subunits of a protofilament before it completes one turn.

Here is a good article on mictotubules.

On the Surface Lattice of Microtubules : Helix Starts,
Protofilament Number, Seam, and Handedness
E.-M. Mandelkow,* R. Schultheiss,* R. Rapp,* M. Miller,' and E. Mandelkow*

Abstract.
The tubulin monomers of brain microtubules
reassembled in vitro are arranged on a 3-start
helix, irrespective of whether the number of protofilaments
is 13 or 14. The dimer packing is that of the Blattice
described for flagellar microtubules. This implies
that the tubulin core of microtubules contains at
least one helical discontinuity . Neither 5-start nor 8-
start helices have a physical significance and thus cannot
be implicated in models of microtubule elongation,
but the structure is compatible with elongation of protofilaments by dimers or protofilamentous oligomers.
The inner and outer surfaces of the microtubule
wall can be visualized by propane jet freezing, freeze
fracturing, and metal replication, at a resolution of at
least 4 nm. The 3-start helix is left-handed, in contrast
to a previous study based on negative staining and
shadowing. The reasons for this discrepancy are discussed.

pdf Link: http://intl.jcb.org/cgi/reprint/102/3/1067.pdf