Cryo-electron tomography and 3D-modeling indicate that the hyperstable ribbon is located at the partition between the A- and B-tubule of doublet microtubules in sea urchin sperm flagella

Doublet microtubules that form the cytoskeleton axoneme are essential for the assembly and stability of flagella and cilia. Genetic defects that affect the assembly of motile and sensory cilia are known to cause human diseases, such as polycystic kidney disease, situs inversus and infertility.  Previous studies conducted on the doublet microtubules of sea urchin sperm flagella shown that Sarkosyl detergent treatment dissolves the doublets with the exception of a specialized set of 3-4 protofilaments of the A-tubule. These protofilaments, named the ‘Sarkosyl-insoluble ribbon,’ are composed of several proteins, including the conserved, structural protein known as tektin.  The exact location of these ‘Sarkosyl-insoluble ribbons,’ has been a controversy for the past 40 years.  To localize the ribbon and tektin protein, we are using cryo-electron tomography and image processing tools to reconstruct the 3D structure of thermal fractionated A-tubules, i.e. partially dissolved doublet microtubules leaving the hyperstable protofilaments.  We determined that the hyperstable ribbons extend from protofilaments A11-A13 which forms the partition between the A-tubule and B-Tubule of doublet microtubules.  In the future, we will confirm by antibody labeling if this partition ribbon contains tektin, which would verify that the partition is the ‘Sarkosyl-insoluble ribbon.’  If tektin is not present in the partition, this indicates that there is more than one stable ribbon in doublet microtubules. We believe the increase understanding of the ciliary and flageullar structure of the axonemes, will provide greater understanding of these organelles in health and disease.