Android OS uses a lot of data in background, how to turn auto system update check off?

I have a nexus 5 with Android 4.4. If i look at data usage, it shows that Android OS is using a couple of megabytes per day. I tried disabling sync and backup but it didn't help. What could be causing the background data usage? One thing that I could think of is automatic system update check, but there seems to be no way to turn it off.

What could be the cause of the data usage and can I fix it without rooting?