Explain Proteins as lubricants?

The mucous of the respiratory tract, oral cavity, vaginal tract and the rectal cavity reduces the irritation which might be caused by materials moving through these passages. This inucous is a mucoprotein, a conjugated protein which contains hexosamine. Proteins as lubricants also surround the joints and facilitate their movement. The absence of these lubricants or substantial decrease in their fluidity through deposition of minerals makes skeletal movement difficult and painful.