Book says threonine has chiral center in its sidechain? How?

[ppppparker]

Text says that threonine and isoleucine have a chiral carbon in sidechain. I am not seeing it.

The carbon in the sidechain that is next to the alpha carbon is attached to an oxygen, a hydrogen, a carbon (a methyl group) and the alpha carbon. So that means its attached to two carbons, so therefore it doesn't have 4 different substituents attached so therefore its not chiral.
Its probably something to do with me not grasping definition of chiral I am geussing, ..
do you consider the atoms attached or is it the substituents that must be different for chirality? For example, would CH and a CH2 be considered two different substits?
thanks

 

[Ygggdrasil]

The substituents must be different, so the methyl group and alpha carbon count as two different substituents.