It's funny because I intended to provide some of the information that u/masklinn (in much less detail, of course), but saw that he/she had already done a superb job. Still, I want to say something, so I'll mention that the particular, slime like feel of the mucins derives from the heterogeneous, O-linked glycoproteins (i.e., carbohydrate chains linked through an oxygen atom to a protein). These carb moieties have a negative charge on them that causes water molecules to be electrostatically attracted to them, making them very "wet" and slippery.

PS: The UN u/Snule is perfect for this conversation. :)