FAQ: Are all membrane and extracellular proteins glycosylated?

No, although glycosylation is very common in proteins that are secreted through the ER and Golgi, not every protein carries the molecular signaling to be modified with sugars. Also, some proteins might exist in different states of glycosylation depending on which tissues and under which conditions they are expressed. A well-known example is pancreatic ribonuclease, which in vivo can be found as its glycosylated (RNase B) and non-glycosylated (RNase A) forms.