Answer;
Hyaluronic acid
Hyaluronic acid are the substances that joins proteoglycan together into gigantic complexes called proteoglycan aggregates.
Step-by-step explanation;
Proteoglycan aggregates are the major structural components of the extracellular matrix of the cartilage, comosed of aggrecan, hyalutonana and a link protein.
They provide cartilage with gel-like property and resistance to deformation through water absorption.