The 3 protofilaments A11, A12, and A13 of the A tubule along with associated inner sheath microtuble inner proteins (MIPs), either in the lumen of the A tubule or of the B tubule, which stabilize these three filiments within the axonemal doublet microtubule. The ribbon protofilaments separate the lumens of the A and B tubules. [ http://www.ncbi.nlm.nih.gov/pubmed/1262413 http://www.ncbi.nlm.nih.gov/pubmed/29430673 GOC:krc ]