"what is called" vs ", what is called, "

[Rei]

Hi,
1 My mother is what is called a walking dictionary.
2 My mother is, what is called, a walking dictionary.

Are these both acceptable?
If so, which is more natural? Which do you prefer?
If so, what is the difference?

Thanks in advance!

[Mister Micawber]

.
Only #1 is acceptable.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber